No idea why it doesn't work with Alexander. I just tried myself and it bugs out for me too.
As for why 25% hp doesn't work I suspect it's because its not really 25% but maybe just shy of 25%.
You need a 25% increase in max HP minimum for your current hp to "become" 80% of your new max HP
For example if someone has 10000 HP, an increase of 25% will make his MAX HP 12500. Current HP stays at 10000.
10000/12500 equates to EXACTLY 80%. So anything less, say for example it's 24.9% and not 25%, will be just shy of triggering the summon blessings.
Fun fact for me. I did some testing with palom calling rude, and kadaj as a party member. My game crashes every single time when palom calls Rude. Had to do the testing without Kadaj for the game not to crash.