Good topic since it is a confusing concept, and it doesn’t appear to be completely consistent.
Another thing to consider the Health Up buff and Health Down debuff. I don’t remember having missing health when either of these wear off.
As an SLKR owner, I have seen Hux’s health bar change colors during raids simply from gaining turn meter (going from yellow to green). That would indicate that he’s losing max health but retaining his current health. And SLKR (maybe others on the team too) certainly gains protection along with max protection when losing turn meter.