I was seeing something like this a few days ago Sigil 0.2.3
CV
I was changing all nbsp; to a standard space (on a single page)
I tried Find (found),
Replace (see change),
Replace All (see changes),
Click back into CV text area
I watched the screen REVERT to (S+R box is not closed)
Set insert point to TOP
Repeat
This time JOY
This was repeatable for 20 different chapters