Looks like cividesk.normalize based on the pastebin.
Actually looks like it's already patched in master: https://github.com/cividesk/com.cividesk.normalize/commit/221562b8bf4fb56fe677eaaf9122d3af5c3cce74
There's a WIP PR (work-in-progress code change) that if I remember right while reviewing it made it mostly work, but I do think html is iffy right now.
You could do it with an extension that just inserts some jquery that appends your text to the existing <div>. https://docs....
I believe this is a regression: follow along here (and do report you're seeing this as well): https://lab.civicrm.org/dev/core/-/issues/1712
And it has been fixed (missed closing it).
You will want to upgrade your CiviCRM to either 5.24.6 or 5.25.0 (we're not sure when the fix went in).