I'm in the middle of rolling back to the Mojave install from the Recovery partition; think I'll do that and stop there, also am going to scour the etc directory for any wonky .conf files I stuck there.
Also reported the bug to Apple via their feedback page, but I doubt I'll ever hear back from them.
Edit 1: Yep, that did it, back to the usual 11-12 GB of RAM free. Phew!
I did it again too, back to normal, and waiting for a fix (I'll report on the Apple feedback too).