I like to listen to audiobooks with mx player and the files tend to be large (>10h).
Very often after I am done listening to the audiobook and pause the playback and switch to other apps and then return to MX player later I can see the progress indicator skip back to the start of my previous listening session.
This happens in about 50% of tries but apparently only after Android has had time to clean up memory. I am not sure on that though. It doesn't seem to matter which Audioformat the file has.
My guess is that under certain circumstances the current playback time is not correctly stored.
I am using a a rooted Samsung Galaxy S7 with the latest stock firmware.
The bug has occurred on my Nexus 5 over a year ago too though.
I can provide more information if needed.