It seems like if I do certain things like merging down subtools, UVs will be deleted. Is there an ongoing list of what all actions delete your UVs in ZBrush. I’m trying to avoid deleting them. I occasionally check to make sure they are still there, but would like to know what is triggering the loss.
There is an option under tool > subtool > merge > “UV’s” that preserves UV’s when you merge your subtools.
Seems like it should be on by default imo. 
Also, you can always re-import your UV’s at any time by using Zplugin > UV Master > “Copy Uvs” from an old model that still has the UV’s and then “Paste UV’s” to your new model that lost the UV’s. The only catch is that your tools vert count and vert order have to be identical on both models.
If that doesn’t work the third option would be to just re-project your detail from your model that lost it’s UV’s back onto an older model that still had the UV’s
Generally any zbrush function that changes your models topology, like re-mesh, dynamesh, panel loops, group loops, slice curve brush, trim curve brush, etc. would delete/mess with your UV’s since your UV’s are tied to the verts, so if those verts are no longer there, then the UV’s are gone as well. I’m not aware of any ongoing list of functions that remove your UV’s though =/
Thanks! I was unaware of UV master, and definitely overlooked the UV button when merging tools. I would Use GoZ and change the Mesh back to one with UVs in Modo then it would project detail when I sent it back. It’s always good to have multiple options with stuff like this.