I believe it’s a new feature that extra reviews are added into an ongoing review session sometime during the hour that they become due.
(e.g. If you start a review session at 10:59am, then you might get a load of reviews added into your current session at something past 11am).
The issue is that the new reviews added in don’t seem to follow the established ordering of reviews.
I could understand if you had to go backwards in the review stages (if say you’re doing reviews “ascending” and you’ve already progressed through “Beginner 1” to “Expert 1”, but then some of the new reviews added in drag you back down to “Beginner 2” before you work your way upwards again from there).
However, after new reviews are added in, the order seems to be thrown off completely and seems to become random.
The only pattern I’ve noticed is that ghosts seem to be clumped together, but within that clump you could get say Ghost 1 → Ghost 4 → Ghost 4 → Ghost 1, etc.