So based on that, there is some issue that is specific to that particular epub or your system (ie interference from AV software?)
Is there any way you can post that epub someplace (privately) and PM me here (KevinH) with the link. I will use it to try to recreate the issue you are seeing and get it tracked down and fixed and then will delete my copy when the fix is in place.
If not, there is a Sigil plugin (Borkify) that will replace all of the text of the epub with gibberish but not change the structure or attributes, so it can be zipped up and posted here without violating any copyright.
See
https://www.mobileread.com/forums/sh...d.php?t=263156
I could use that and edit the gibberish and replace it with words like "favour" and "colour" in a couple of places to try to recreate what about this epub is confusing Sigil so much.
Thanks!