Just now, we noticed something strange again for another blog post. We surfed to the URL (hidden link) and saw it was in Dutch (correct language). Then, we clicked edit, and it literally switched to English in front of our eyes. Luckily, we could use revisions to put it back, but the "editing" triggered the overwrite when originally the language was Dutch...
I think changing the setting helped in not overwriting when clicking edit, so that's great. Thank you. Regarding the other issue, I don't want to close the ticket just yet as it hasn't been that long to see if it's fully resolved now. I will keep monitoring the automatic overwrites this coming week and if nothing strange happens anymore we can close the ticket.