You'll have to search out the various threads where kindlebook designers are beating their heads against the wall trying to get that feature to a) work consistently, or b) not get changed after uploading to KDP. There's nothing specific to Sigil that can be done about it though. You'll just have to try one of the numerous "this works for me" techniques out there and see if one works for you. Check the kindle formats subforum for ideas. This one was a popular one:
https://www.mobileread.com/forums/sho...d.php?t=191387