OK, this looks (for now) fixed. I have gone back to 535 and now resume is working again.
What has made the difference, I guess, is that installing 535 has restored /lib/systemd/system-sleep/nvidia. Deleting that file used to be part of the procedure to disable the systemd suspend/resume mechanism in favour of the kernel driver fallback. It looks like this is no longer the case and that the file needs to be left alone.
OK, this looks (for now) fixed. I have gone back to 535 and now resume is working again.
What has made the difference, I guess, is that installing 535 has restored /lib/systemd/ system- sleep/nvidia. Deleting that file used to be part of the procedure to disable the systemd suspend/resume mechanism in favour of the kernel driver fallback. It looks like this is no longer the case and that the file needs to be left alone.