For what it's worth I've been away from Minecraft for a long, long time, but I have used Botania long ago, and had a similar setup that worked just fine. I get that the loonium spawns enhanced mobs but I don't remember it being like this:
I'm playing on a friend's server (1.20.1) and I started up my loonium with a hyacidus next to it. It spawned a spider with fire resist and regen that wouldn't die for a very long time despite me pounding it with a fully enchanted thundercaller. For some reason it would lose health and then the health bar would just jump up like it drank a healing potion. I have a five minute long video of me swinging away at it and at some point it finally died, but there were also two similarly enhanced skeletons, and not only did those never die no matter how many times I hit them, I realized that my Loonium and hyacidus were gone. Did their arrows somehow destroy them? Why did they keep regaining health so fast? I get that they had regeneration but surely they should be killable at least, right? What am I missing?
At first I thought maybe it had to do with the thundercaller, so i switched to my Terra Truncator but that didn't seem to help. Then I thought it might be the poison effect of the Hyacidus but upon glancing at it I noticed that both it and my Loonium were gone.
???
edit
Okay so removing my charm of the diva made them no longer immortal. Not sure why.
Any idea what happened to my flowers? Did the arrows pop them somehow?