FWIW, I've been running into those issues some more, but going into SYNC and quickly trimming out the aircraft fairly reliably "fixes" autopilot behaviour down the road (and the glideslope). Basically it starts happening when slowing down during arrival or approach, I trim the plane once, and it'll keep working the rest of the way.
I hope the RJ received some better logic there and we see it ported back soon :-)