Better indication of "near death" condition

I'll definitely be looking to improve the overall feedback that the player receives related to the status of their health. In theory it should currently function pretty close to what you've described, but there still may be some unintended interaction between the survival stats that is resulting in the confusion you are experiencing.

After re-watching your developer video about the health/vitals system, and assuming it still works more or less the way demonstrated there, I think the problem is that there's a fundamental conflict between your concept of what and in what way the health system should tell the player, and what the player unavoidably needs to allow them to utilize their health level as a survival resource.

You wanted to avoid representing health itself as a simple numeric value that just goes up and down, since that's not how it works in real life, and I do agree with that, it's a nice idea, at least in theory. So instead, you created a number of "healthness factors" to manage, which represent your body's ability to maintain (or lose) its "health" over time. This is clearly visible in the video, as you modify the hunger and thirst conditions: a hidden health value starts dropping more and more rapidly over time the worse those conditions get. This is what the 4 displayed vitals categories actually represent, and what I think is the primary source of confusion: they are not "flat" or direct health indicators (like, if you have 3 red lines in hunger, you are missing, say 30% of your max health, and if everything everywhere is red you die; no, you just lose a hidden health value more and more rapidly), they only show the rate at which you gain or lose health. But your actual health itself is not represented in any way, only these "health changer" factors are, which do not and can not tell you anything about how long can you last with the current health loss rate.

It's like flying an airplane with only a variometer but no altimeter: you know how fast you climb or descend, but you have no idea about how far the ground is so you don't know when you have to stop your descent not to crash. And while it would be easy to say "just never allow negaive RoC and you'll be fine", that's just not possible.

The problem is that from a gameplay perspective, the player has to be able to have at least a rough idea about how long it will take for the character to die due to the effect of the "health changer" factors, to be able to make decisions like "can I keep going towards that sensor ping, or I have to turn back now as my health won't last long enough". And I don't see any way to avoid some kind of "flat" health indication to be able to make such decisions as the health change rate alone can't give you a "time left to live" when you don't know the value the rate is applied to, and currently you don't.

That's why I think what I wrote previously is what the vitals system could (should?) be reworked into. You have the 3 "tiered" health changers: hunger, thirst and exhaustion (weighted, so that hunger gives the highest delta per tick, thirst gives a moderate one and exhaustion gives the lowest, set in such way that it's not possible to compensate hunger and thirst both being in the red by just sleeping). You have 2 "direct" health changers: accumulated radiation and body temperature, low temperature acting much more aggressively than any other factor. The positive cap of the tiered modifiers (all green) should be somewhat lower than the negative cap (all red) so that it takes longer to regain health than it takes to lose it, ad no health gain from no radiation nor acceptable temperature, those can only degrade health or not, but not improve it. And then you have "<quality> Organ Integrity" which is the representation of that previously hidden health value which all the above modifiers decrease or increase over time. The modifiers show you in which direction your health changes over time and allow you to influence/manage this change. Organ Integrity shows you clearly how far away you are from fatal organ failure, which means death right there and then (currently it means "nothing", except being another rate changer, as at some point in the video it drops to its lowest value, yet the player character does not die), but you cannot directly modify it by flat values (but there could be a medical/medicine mechanism, which slightly boosts the recovery rate on top of what you get by keeping the modifiers in the green). When the modifiers are already red but integrity is still "greenish", you can risk keep going. When the modifiers are red and integrity also starts turning red, you must improve the modifiers or you will die soon. This would also give a long term time delayed effect to overstressing your body: go with red modifiers for too long to have integrity to drop to orange and for the next few days you won't be able to go as far as you did before, since integrity now starts dropping from much a lower value than before unless you allow yourself to recover first.

The screen itself should also get reorganized a bit to communicate this. Organ Integrity should have its own line either at the top or bottom, as if saying "organ integrity is influenced by the below factors" or "organ integrity is the result of the above factors" (you could further emphasize this by making a boundig box with organ integrity as the "title" of it, which box then contains the 4 modifier group boxes), and the 4 groups could remain and work the way they are (with their boxes slightly made smaller to make room for the organ integrity line, and Exposure missing Organ Intergrity due to having it repurposed).

(I hope you don't mind the wall of text, I'm just trying to find a middle ground between what you have in mind for health management, which I think is a great idea in general, and what I think would be needed to better support gameplay considerations at the same time.)