Not entirely sure about its replicability - but I have just noticed the same as @shahram7 in https://forums.zotero.org/discussion/comment/473250#Comment_473250 that the last page read is not synced - though all the annotations are.

opening up a new thread as requested by @dstillman

Additionally, it'd be nice to know the mechanism for syncing on androids as it seems to be only triggered manually? I think this is the case for the library - not annotations (which works instantaneously!).