technology Iphone doesn't see audiobooks anymore

[Light Blade 8,673]

So I just discovered that my iPhone lost all of the custom audiobooks that I had on it. When I tried to sync them again from my Macbook, the Audiobooks section of the iPhone sync menu showed me that there are no audiobooks to sync. However, I re-checked the Books app on Macbook and not a single audiobook went missing. With that said, I can no longer sync my audiobooks on my iPhone and I have 2 facts that contradict one another. Did anyone encounter this issue?

[Jeric 46,851]

You may want to reach out to Apple directly on this as there have been reported issues for some time with Audiobooks that were not sourced through their ecosystem. Ever since Catalina IIRC. Off the top of my head try and see if they are in 

Users/"username"/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks/Books

This is where my iBooks purchased audiobooks are stored on my MacBook. 

[Light Blade 8,673]

@Jeric Already checked that. All my audiobooks are still there and can be listened through desktop version of Books. It is my iPhone that stopped seeing them.