I've reported this bug before in another thread paired up with another bug report -- Thankfully that one issue has since been fixed, but this one bug is still lingering and has been around for awhile now. FYI, running an iPhone 6 with the most recent iOS version and the most recent Plex iOS app version.
So, I use the Plex iOS app to play my music from home. I have a 2015 Honda Civic and I connect my iPhone to my car via the USB cable (I personally don't like using Bluetooth for music).
So let's say I hit play on an album that has ten tracks. When I hit play on the album, the car displays Track two, when clearly it's the first track being played. The artist/album/track name shows up correctly. However, once you get to the actual last track on the album, the cars display shows track eleven out of ten and doesn't display any of the artist, album, or track data.
Some screen shots:
Here is the full album, as shown in Plex: http://i.imgur.com/UVNcf0q.png
Here is what the car displays when I hit play: http://i.imgur.com/ALXUpXe.jpg
Here is what the car displays for the last track: http://i.imgur.com/ypIy2g2.jpg
Let me know if you need anymore data than this. Happy to help beta test the app while you (hopefully) work towards a fix!