when the player has non default health values (e.g attributes, stats, heart containers, etc) regen does not correctly account for these, causing the player to regenerate erroneously.

More info? Such as what mod adds this health etc?

there are a fair few. one called "diets". another called "spice of life: carrot edition". apotheosis adds "afix" tools and armour with buffs like increased health. baubaly heart containers (originally an addon for azanor's baubles and updated for curios. also not sure about spelling). um... absorption hearts were also seeming to be ignored. i will let you know if i think of any others in my pack but the thing they all have in common is that i do believe they affect the player health attribute directly as opposed to a separate buff. the thing in question that triggered a regeneration was heatstroke from "cold sweat" so its possibly an issue with that triggering an erroneous death.

how exactly are you handling the regeneration detection? are you implementing your own health check or bypassing the vanilla health check to include a branch case for regenerations?

With the new changes, this issue should no longer be a problem as only when the player actually dies, does regeneration kick in.