More info:
Apparantly the issue was addressed a long time ago in Oct 2014, Maybe the bug fix back then didn't work out as planned.
see

https://ubbwiki.com/wiki/view/37/ubb-threads-v7-1-1-changelog.html

9th bullet point under "Bug Fixes"

Fixed a bug with the Active Text featuring causing problems when the text fell within image tags.

Maybe this could be a lead

Morgan Johansson
BritBike Forum
https://www.britbike.com/forums/ubbthreads.php