This was the case with hidden faces and `Sync Selection` turned ON.
Caused by 8f543a73ab.
Since 8f543a73ab, the UV element map
respects the hidden state of geometry, but stitching [which also
respected this on its own even prior to the culprit commit in its
calculation of connectivity] did this differently [it only skipped
hidden geo when UV_SYNC_SELECTION was OFF -- even though UVs would not
be visible which is probably the real error here, I believe there is
this principle that we "dont act on stuff we dont see"].
To fix this, also skip hidden geo (even with UV_SYNC_SELECTION = ON) in
the stitch calculation of connectivity, just as
`BM_uv_element_map_create` does it.
Philipp Oeser