Hi!

Not sure if I understand 100% what you mean, but both of those things work on a round basis. The standard is wave 1, wave 2, boss (or wave 3), so if you increase or decrease enemies max.hp during the waves, the effect dissapears when you move on to the next wave. The boss is just a long round, that's why "it stays".

hope it help!

I believe OP is referring to losing max hp across rounds to the enemy (skull lord) but champs like rotos (others?) not maintaing the gain of max hp from rd to rd.

Thanks Trips, that is what I meant. Just to put a lot more words to it:

Attacking Skull Lord is a great example in DT waves as you always suffer some max HP reduction since it is a passive ability. Then when you move to the next round the reduction remains. 

Conversely, when my Rotos increases his max HP it will reset before the next round. 

Very annoying that there is no consistency unless I am missing something.