I think this is a core bug, but not 100% sure.
I have a site with 3 languages.
I had a node in the original language and 2 translations, each with a different URL alias.
Then I cloned the original language node.
The result was a cloned node + 2 cloned translations.
I was able to edit the cloned node and translations, except for the translations' URL alias. Their URL alias field appeared blank (expected behavior) but I was unable to save new URL aliases.
I hope this description of the problem is clear enough.