This is going to be frustrating for you Steven, but it doesn't appear to be fixed. The mixer came up with the following set of tracks after I'd updated to a recent version.
As you can see, the highlighted tracks are missing from the Now Playing list which is what the mixer gave me. The one link I can find between the missing tracks is that the first three have the 'Keep in sequence when shuffling' tag checked, meaning they'd play as a four when being shuffled as standard. Does that help trace it down?
It's also worth noting that it appears to be the auto-playlist the mixer draws from which is causing the problems. I just refreshed through it a bunch of times and eventually got an occurrence. As you can see, it's only picked up one track from ELO's Time album, when there should be 13.
When I initially spotted this bug two years ago, I posted a long thread in the General forum. Link here
if it provides any help or clues. Notably, seems like I decided the 'Shuffle by Album' setting had an effect on whether or not it happened.
EDIT: Happy to run a debug version as always.
Bold words in my posts are links unless expressly stated otherwise.