I have Windows 10, so I had to use a workaround to get TFN 1.3.1 running. I remember that this never happened with my previous Let's Play attempt, which was done in Windows 8.1. Also, I noticed that the life bar randomly disappears in UP 9.4+ as well, so this might be a problem with Wesp's Unofficial Patch.
It's only been a problem in TFN and not in the UP+ because my character takes HP damage when his blood pool is low, and sometimes I enter combat without knowing this since the life bar is too faint or outright gone. Also, combat in TFN is much harder and more lethal than in the base game, even with the UP+ installed, so it's a bigger problem now.
However, it's not a game breaker, as I can simply outlast my enemies with the innate Salubri regeneration + Obeah 3. I can live with it, but would prefer it to be fixed. Not sure if you can fix it, though, since you don't seem to be on Windows 10 (judging by the last few posts of yours that I read). You might need someone who has Windows 10 to find out how the bug can be patched.