If all the changes are are insertions of a space between run-together words, it does not save them unless some other change is made.
My book has been full of sentencesthatcontinueontogetherwithnospaces in between. I have been spending a lot of time breaking the words up, but it does not save them all the time.
I suspect this is why Sigil inserted non-breaking spaces whether you wanted them or not.
ACK. Dragging words causes a crash. I wrapped words which had no paragraph tags in paragraph tags. I went just below and selected the footnote which had been left out and dragged up then crash!
split 9 line 271
Tried the same thing by deleting space between end of paragraph and footnote, dragged <p> tag to beginning and </p> to after footnote and it did not lock up.
Attaching the document in question.