I gave a boss monster with fast healing this spell without thinking through the implications, and then realized a day's worth of rest would mean fast healing would put her back up to full. That didn't strike me as an intended use of Moment of Renewal. Obviously, an easy solution is to not give me monsters this particular combo, but there are a few ways PCs can get fast healing as well and it IS an 8th level spell. But monster's higher ability scores also means it will get a lot more healing than an equal level PC.
What do y'all think?
I would not apply fast healing and regeneration when someone casts Moment of Renewal. Both effects speak of a "beginning of your turn" which doesn't happen during Moment of Renewal. I would just apply the rules about 24 hours of healing (so an 8 hours rest + all the benefits happening after 24 hours). Even if it's illogical, it would be too much of an interaction to allow that. Healing 6 creatures up to full is far too powerful in my opinion.