When you use the ability on a mob and bring it to 0 HP, the mob becomes invulnerable to all damage and can no longer be killed by any means, including undead mobs in sunlight.

When you use it on a player....

I was easily able to replicate this issue with only the bare minimum amount of mods installed:

This happens 100% of the time when dealing the finishing blow using the Skyward Sunder ascendancy ability to any mob.

Mods installed when testing:
simplyskills 1.5.0+1.20.1
cloth-config 11.1.118
player-animator 1.0.2-rc1+1.20
puffish_attributes 0.5.0
puffish_skills 0.12.2
spell_engine 0.15.8+1.20.1
spell_power 0.11.1+1.20.1
trinkets 3.7.2
jade 11.9.2+fabric

Same issue with Cyclonic Cleave and Ghostwalk, entity hp gets set to NaN upon any hit with those abilities (can be verified with, for example, CraftTweaker's /ct entity_info) - doesn't even have to be a killing blow.

When used with "Armor & damage scaling" mod this also causes a tick crash.

Really hope to see a fix for it, since that makes the game effectively unplayable for players with those abilities.