one of two things, or maybe both, manjaro has a really weird issue with the AUR where they repeatedly pushed updates to pamac that have crippled the AUR.
there are also often times where AUR packages inexplicably break on manjaro so using the AUR while running is is fairly sketchy
I think I understand your latter point. I had no time to invest in another arch build last time I did a reinstall. This was before arch had a gui install. So I went Manjaro. I've noticed this problem where sometimes I install direct from aur and it messes stuff with pamac. Now that there's a gui install for arch I'll just go back to that next time.