I confirm I don’t reproduce anymore if I add
config['xwiki-upload'].isTemporaryAttachmentSupported = false
in the wysiwyg editor configuration in the wiki administration.
So to me this is a bug of the xwiki-upload plugin, of this new temporary attachment feature.
Reproducing the problem is very straightforward to me, on any translated page - e.g. on Sandbox.TestPage1 with the wiki in french. It reproduces all the time.