Same problem here. First the audiobook titles (all from Audible) showed up, but would not play (I was in airplane mode at the time). Then when getting out of airplane mode, my iPhone 6 (IOS 12.1.2) shows no audiobooks, despite iTunes showing them all and claiming it synced them to my phone. Unbelievably annoying. I got past it my using the Audible app instead of Apple's book App, though it had to redownload everything. so now they are likely there twice with one copy hidden in some way just taking up space.
Tim Cook lowers forecasts of iPhone sales and says it's due to fewer upgrades from battery replacements, China's slowdown and trade tensions with China. Of course it could not possibly be that folks are becoming fed up with the ever growing list of sw problems.
It used to be that this sort of thing didn't happen (at least often) as Apple controlled all the software. Now it happens because Apple wants to control if not provide all the content too. What a surprise that this issue doesn't happen for books purchased from Apple.