If anything it tends to be a yellow heart. Either way, if you have 2+ mods installed that handle temp hp differently, this sort of conflict can happen.
Off the top of my head, gremlins (now in downfall) and dragon tamer implement temp hp. If you're the type who has all mods loaded at all times, check the compendium for relevant cards and remove their mods' checkmarks so only the mod you want to play handles temp hp.

Zu, yes, yellow, and yes, that's probably where the conflict is occurring.
Too many modded characters active, going to have to turn them all off.
Then test and activate one by one. This is going to be painstaking.

PS, not sure why I thought/remembered pink.

I think that KomeijiMod is causing the problem ... posted a bug report
When I removed KomeijiMod from the active, temporary HP became visible again
https://steamhost.cn/steamcommunity_com/workshop/filedetails/discussion/1613792088/3079880882840266301/

Likely. ModTheSpire keeps getn updated while Komeji hasn't been updated in 1.5 years.