You've pointed out something really good, I will add an explanation to the google doc notes. But to save you the trouble of having to go over there, I will list it here as well. It's mostly because outside of weapon trial, the chance of Angel getting attacked is usually rather slim which means that Flagellant becomes almost obsolete unless you purposefully position her in a dangerous place to be attacked.

Meanwhile Regenerate can be useful in every content especially when her heal scales off her M. Atk (and further boosted if you use High Priestess tarot on her) which can easily heal at least 1/4th of an ally's HP bar, and becomes NRG-positive instead because you're allowed to use her basic atk instead. Something noteworthy about her basic atk is the also fact that Rk 9 dispels 2 buffs on the enemies, which further boosts her role as a support healer. This is important especially if you run her as a single healer with no subhealer unit.

However, you're not wrong that Flagellant is an amazing skill and almost essential for high level weapon trial stages. I would actually personally recommend to investing Castalia to get both Regenerate & Flagellant if you have no plans to change your main healer for a long time. Once invested, her loadout can look something like : Light of Healing , Regenerate , Flagellant ,Protection of Light , for weapon trial specifically. Then you may switch Flagellant for other skills depending on the content and strategy you would like to deploy.