[the_scotsman]

Thanks, so this turned out to be something so sily.

There's a plastic outer collet thing around the bottom of the shifter. This had unclipped itself from the shifter handle. And it had partially dropped down into the shifter mechanism, under the shifter cover, and it must have been fouling on the mec hanism. When I pulled it out and clipped it back on, the issue was completely resolved.

Crazy, but glad it was an easy fix!