You always have to expect issues when using multiple plugins at the same time that all mess with the render pipeline...like multiple PostMagic being active at the same time, interference with Reshade, etc.
However, there is a know issue with PostMagic and disapearing UI. Several people reported it over the years:
MacGruber submitted a new resource: PostMagic - Experimental post-processing effects for VaM PostMagic adds post-processing effects to VaM desktop, partially also to VR. Some of the effects have issues or are even marked as experimental. Before you get your hopes up, this release is identical...
hub.virtamate.com
However, PostMagic can't be improved or fixed. I would have to implement new post-effects from scratch, since off-the-shelve effects won't work when applied as VaM plugin. Would need hundreds if not thousands of hours which I just don't have. For VaM 2.0 Meshed will likely be able to implement some off-the-shelve post-effects in a fraction of the time.