I am running the larest firmware on the basic non-pro version I picked up in early April this year. First thing I did was update the firmware.
One thing I also do when I rip MP3 files is to set the MP3 tag properties in Windows/File Explorer. I set the track numbers to sequential numbers from 1 to however many the total number of tracks are in the entire book.
It’s entirely possible that the Dash is using the track number and not the file name to establish the play sequence. I recall that the reason I needed to set this was because one of my phone media player’s sequence was driven by the track number. Again, I used to do this all with keyboard shortcuts. I believe F4 pops the file property sheet, tabs get you to the tags page, a couple tabs to the right field and then overtype the track number (or F2 to select the track number in edit mode and key the track number).
It’s been a few years since I’ve ripped a book as a have a few hundred on tap so I can’t recall the specifics but again, keyboard shortcuts make it a lot quicker and less tedious than working a mouse.