@jpomart - another long-shot tangent. There's a widely used Word add-in that creates EPUBs from Word documents - it has these options - no idea of there significance or if its relevant to the reader -as I don't use that bit of add in.
FWIW I edit epubs in Sigil - and I have no trouble reading with calibre ebook-reader - just works, but I've no idea why.
BR