Hello zsolt d,
First things first - this changeset did remove the member ways from 23096 - you can see that by clicking on the "osmChange XML" below and looking at the end of the XML.  You can see that the last few relations have no members.
This doesn't of course, explain what happened.  I'm sure that it was not malicious (so suggestions of that by balint36 aren't helpful) but something did go wrong, and if you don't try and find out what that was, it might happen again.
When you upload with JOSM you do get a summary of what you've changed.  Maybe it would help to upload more often with fewer changes in each changeset?
Best Regards,
Andy
