This one's pretty straightforward. If you have 100/120 HP, and you eat The Prince, you now have 112/132 HP. When The Prince wears off (assuming magically you manage to take 5 hits without losing any HP), you will be at 100/120 HP again. If you have 100/120 HP, and you eat a magical mod mushroom that gives you +6 Burliness and +6 Caddishness, you will once again be at 112/132 HP. But when that mushroom wears off, you will be at 112/120 HP. You lose HP when you lose +HP buffs, but you don't lose HP when you lose +Burl/+Cadd buffs. It doesn't particularly matter which way this is corrected in the long run, but it should be consistent between the two. Personally, of course, I vote for the way that makes The Prince not something Admiral Ackbar would make dramatic statements about, but that's just me.
Simple solution: Turn the HP into a permanent, 'Phantom' buff which lasts for 15 hp worth of damage, and then dissipates once 15 damage is done to the player from whichever source. If that's possible in the current coding scenario that is. Stupid solution: make the mushroom 'permanently' add +15 hp, and once damage taken = or > 15 add a debuff to lower to your max by 15 but still retain original player life values. Probably needlessly complicated.
I agree with Essence on both points: This should be handled consistently. Players should not lose health when a +HP or +Burl/Cad buff wears off. However, this leads us to a third case, which is worth discussing: If you suffer from a curse (lingering weakness, for example) which reduces your max HP, what should happen when it wears off? Currently your HP jumps back up - which is why your HP jumps down when the Prince wears off. Are we willing to say that when -HP debuffs wear off, your health remains low? For example, you have 50 health and get hit with a -10 HP debuff. You regen 2 HP while the debuff is active - so now you're at 42 when the debuff wears off. I'm suggesting that, to be consistent, you just stay at 42 HP instead of jumping up to 52 at that time. Thoughts?