I am an author who is keen on using open-source software to write with. When LibreOffice 6 came out with EPub export, was very excited. When I exported one of my shorter manuscripts to EPub, however, I noticed (to my discomfiture) that the chapter numbers were missing. Since I use a field for the chapter numbers, this is of immediate concern. How to correct without repeating work, or (worse, in my opinion) hard-coding the chapter numbers?
Export to ePub is a new feature and bugs may be present. File a bug report at Bugzilla. Meanwhile, try exporting to FictionBook2 using OOoFBTools.