Steps to reproduce:

1. create new actor;
2. set their HP using the HitPointsConfig dialog;
3. try to apply healing damage.

Seems like the issue is undesired precedence in this condition:

damages.amount < (this.system.attributes?.hp?.dt) ?? -Infinity

where the < is evaluated before the coalescing. When setting the HP on the actor, dt becomes null (instead of the original undefined), and funnily enough -1 < undefined === false but -1 < null === true. In any case, just fixing the parenthesis should fix it. I'll make a quick PR in a few.