that definitely does not sound like a max error, but something on your behalf. i assume you do not collapse your UV modifier down, then do stuff to the mesh and then your modifier doesnt apply anymore?
So you have two objects selected, and a single (instanced) Unwrap UV modifier added to both of them? Or you move from one object to the next, editing UV's & collapsing the modifier on each one separately? Is whichever material or shader your viewport is using set to the same UV channel that you are working on in the Unwrap…