Back in the late 60s/early 70s it was sung in South Wales to the tune "Austria" (aka. Glorious Things of Thee Are Spoken/Deutschland Uber Alles). This had the merit of having enough notes to allow the whole of the verse and chorus to be sung.

I recall a group of us singing it thus in a bar in Croatia in 1968, and being delighted to find a party of Germans who knew "Clementine" and joined in with us

I imagine it morphed to Cwm Rhondda by accident: both tunes start in a very similar manner, and if you start the wrong one it's tempting to keep going and to finish when the tune does, by replacing the "Thou art lost and gone forever, Dreadful sorry Clementine" passage with another "Oh my darling Clementine".