Okay I'm not sure if anyone else is still having this issue in 2021, but i was, and i think i figured out a solution. Works perfectly for me, and hopefully this post helps others.....
So in the gallery that keeps resetting to no selected items after a successful Patch(), i set the onselect property of all items in the gallery to:
Select(Parent);Set(GallerySelected,ThisItem.uniqueIdentifier)
THEN.... i set the Default property of the Gallery to:
LookUp(DataSource,GallerySelected=uniqueIdentifier,ThisRecord)
Final Step.... the button that is triggering the Patch, at the VERY end of that formula, do this:
Patch(blah blah blah);Reset(Gallery)
Like i said, this is an old issue, but if anyone else experiences it, this workaround works like a charm 🙂 Happy Apping!