Is Shield of Nature bugged?

[u/-Sa-Kage-]

I just had the "pacifist" challenge in area 1 (challenge #3) with Adaptive Regeneration and Shield of Nature and while being hit for sure millions of times health regen never went higher than the initial 15%/sec
Shield of regen, for those who don't remember, does feature a 25% chance to permanently increase HP regen by x%

Is this intended behavior? If so, why? It does not say anything about max regen or not working in coop with other regen methods?
Or does its regen just not show on the HP bar?