I am playing against Bahamut with the Heal Sponge node. I removed my willpower mastery but I have Nimrod, The Champion and Apocalypse with me. When they regenerate, Bahamut regenerates as expected but he can go through the threshold of a phase.
Also these champions are 6-stars so the adrenalin if I get a hit and revering life, it regenerates the defender but also makes him go through the 10% barrier.
I am jumping from Phase 3 to Phase 4 and back to Phase 3 (from 9% to 12% back to 9% so ping pong sometimes, and sometimes no ping pong) for some time now.
This is the node.