UV artifacts after join 2 mirrored (+UVedited) meshes


When joining two meshes (with the same material and UV) one of these (typically the first one) will break its UVs. This will happen only if the mesh UVs are modified on UVEditor AND, after that, mirrored AND joined.
I can’t understand why this happens.
For this time I managed to mirror the mesh before modifying any UVs, join, and after all edit both UVs sets together.

Do you know the logic behind this? Am I mistaking something during the process?

Ps: I found this error in the past, but I did not remember all my actions.

Thank you,