Here is a challenge, seems simple, spot the difference. but

here are two identical meshes 1 and 2. same vert, edge, face count and even the same vert index number.
when use “transfer uv maps” from 2 to 1, 1 as active object. mesh 2’s uv is messed up. vice versa.
if duplicate them and use transfer uv maps operator on their respective duplicates, ( after alter their UVs a little bit) the results are as expected.
so what gives? how mesh 1 and 2 differ?
here is the file UV map transfer.blend (724.7 KB)