Some VST instrument tracks now making no sound

Hi, never had this before, but during working on a purely VST based instrument track project, some (about half) of the 8 or so tracks ive created so far have ceased to produce any audible sound output. They were working, but while the project was open, they just stopped for some reason. The other half still do output sound. There are no audio files of any kind in this project, either as samples or as recorded parts.

Weirdly the silent channels are showing activity on the level meters, just the same as the audible channels. Things I have tried include switching the audio driver, the sample rate, the bit depth, toggling the track solo and mute states and the global solo and mute states but no change.

I’ve rebooted everything and re loaded. The way I work is to save with slightly different file names on each save I make, and if I load a version of the same project from several versions before the current one, the affected tracks play fine. Nothing I can change in the project or audio system settings has made the sound come back on the silent channels. I can’t find a similar example online - either sound output works on ALL tracks or it doesn’t it seems. Curiously if I export the track as a stereo mix all the tracks are present in the exported version, but will not make sound otherwise??

Any advice would be much appreciated.

Cubase Pro 12 - Version 12.0.60, build 453, built on March 16 2023
Windows 11 Pro on brand new PC build, 64 bit
Processor - 12th Gen Intel(R) Core™ i7-12700K 3.60 GHz
Installed Ram - 64.0 GB (63.8 GB usable)
Hard Drive - Samsung NVMe 980 Pro
Behringer Uphoria UMC1820 Audio Interface - Driver version, 11/05/2023