As far as I know it is the TinyMCE library that removes the tags, not XC5 itself.
It was reported to the XC5 team, but no progress was made on that ticket since then (I think this issue has a low priority as the team is focused on the 5.3.x version and fixes only major problems that affect lots of 5.2.x users).
However, you can fix it by creating a custom module and adding extra parameters to the tinymce.init() function as was suggested
there or
there.