Hey Rob,
Thanks very much for coming back to me so quickly and helpfully. I ran your test and the rotoscope rendered fine, so obviously I'm doing something stoopid. It's amazing how much I've forgotten about A:M.
I'm seeing quite a few issues that look like screen driver problems (I'm running a GeForce RTX 2070, fully updated), like the display jumping to a previous state when I drag a lasso, then reverting when I release, and I thought maybe the rotoscope failure was a similar issue. Clearly it ain't, it's just me not doing it right.
I'll be fine now I'm sure!
Thanks again, your help is much appreciated.
Jem