Editing a translation page and saving results in editing conflict with English version

There seems to be a mixup when editing a translated page when the english version of the page has also been modified.

Step to reproduce:

1/ 12.9 install
2/ Setup multilingual english/french
3/ Go to Sandbox.TestPage1.
4/ Make sure to be on the english version of it
5/ Edit english version and add at the end on a new line "Change in english"
6/ Save and go to french version
7/ Edit french version and add at the end on a new line "Modification en francais"
8/ Save

A conflict box shows up mixing the english and french version.
See screenshot.