regen is triggered on player death instead of allowing a totems of undying and/or (modded) items of the same function (if available) to do their thing.

this is possibly an incompatibility with inventory totems (does exactly what it says on the tin)
https://www.curseforge.com/minecraft/mc-mods/inventory-totem

and the soul cube from the doom mod (when in the curios charm slot works as a 5 use totem of undying)
https://www.curseforge.com/minecraft/mc-mods/mcdoom

@Jackiecrazy Did you address this in 3e0fc32 ?

Yes. I am quite sure LivingDeathEvent is only called after totem checks, and that is the only point at which entering grace is handled.

After asking @Jackiecrazy to test the latest commit and then testing myself; I can confirm that Totems of Undying will take priority in the next update of Regeneration.
It is worth noting also, that /kill will override this functionality from my own testing, as will using the Force Regeneration Keybind.

When the next update releases, I will link the download to the file below. This may be a few days away as I am working on other things and some additional features for Regeneration.

To add on to that, /kill does not proc totems as it deals void damage. Force regen damage is also marked as bypassing invulnerability, so it follows the same behavior.