I must apologize for being obtuse.
After looking into this report and this issue #11239, it looks like v8.3.1 is doing something weird with the UDL comment entries, but worked properly in v8.3 and earlier. I now think there was a regression in the UDL Comment handling in v8.3.1. Given the description you showed above – especially with the XML showing single-digit rather than double-digit in the tag value – I believe the problems you were having with Comment Line syntax are caused by the same issue.
Hopefully it will be fixed in the next release of Notepad++ – until then, stick with v8.2.1. And again, sorry for not groking what you were trying to say.