OK, I have found this setting. It's a new one, only here since the last version upgrade of the software, that's why I didn't click.
QUOTE Merge member's concurrent posts
When a member posts twice within n minutes, once after the other in a topic they will be merged into one post. Enter the number of minutes to capture concurrent posts. Enter 0 to not use.
So I've disabled it. It was set to 10 minutes.
Remember that you have 60 minutes where you can edit and add to your post. After the 60 minutes the post is fixed. If you need something changed, ask a moderator.