I've had this problem on and off for some time , but it's getting a bit annoying.
I have a book with dleiberate space lines in it: say (<p class="calibre8">& nbsp;</p>) or (<p class="calibre8">& #160;</p>) as a deliberate space line. At some stage I return to the page and find that the space symbol has gone - which means the line no longer produces a space once in book view!! At the same time ALL other space symbols have gone from the whole document . . .
It
appears to happen randomly even though I know it can't be the case. I have Sigil set to "automatically clean......." on 'open', and also set to 'pretty print tidy'.