This is a bit strange. At least if you modify the entry it should be saved using the “new” style without camel casing… And it should definitely not be an OS issue.
What happens if you change an entry in the BibTeX source tab and then save it? Does it still show camel case in the tab? Or is the camel casing happening only in the saved file, not in the tab?
The problem got resolved in meantime.
It seemed there was a connection with the chosen language. I had Jabref in English, and my collaborator in German. We both updated to newest version and are now using English GUI language and the problem did not appear again. However, I did not test if the update or language resolved the issue.