Hi !
I work on model with something around 30 000 polygons. I use a UV master and work on a clone.
When I copy UV from a clone and try to paste it back to my original mesh (with a polypaint) PolyPaint dissapears…
I really don’t have an idea why 
Please help me if You have any idea how to solve this problem !
Which version of ZBrush are you using?
ZBrush 4R7 64-bit
I tried on the other meshes mostly with lower resolution and it works - so… is it some kind of bug ?
Make sure you have the P3 update (it will say top left of your UI).
I don’t know why it should be a problem with one mesh and not others. It’s not a bug that I’ve heard of. Check that Colorize is still on - the polypaint may be simply hidden.
Thank You for Your help! I will try to fix it 