I had a look in the registry, and noticed that although the Tortoise overlays begin with 2 spaces (presumably in the hope that they are listed first alphabetically and therefore the ones used in preference to any others installed), it seems that Microsoft are also playing this game.
The OneDrive overlays have 5 spaces at the front of their name. Also, Google Drive overlays have 4 spaces.
I modified (via a Rename in the registry) my Tortoise overlays to have an extra 4 spaces (i.e. 6 in total) at the front of their name, and using Task Manager, restarted Windows Explorer. Fixed.
It would be nice if MS could fix this somehow by extending the number of possible overlays past 15. They say it would cause performance problems if they did, but that just sounds like a redesign is required to me.
In any case, it would be nice if the Tortoise project would consider increasing the number of spaces in front of their overlay key names in the registry. I suspect that when I upgrade Tortoise, I'll have to do this again...