UR816C producing 'glitchy' clicks - is it a hardware bug?

I’ve got the UR816C and i’ve found what seems to be a really odd bug.

When playing some vstis ( and it doesn’t happen with all) I get an audible click through the speakers just after stocatto type synth sounds. It kinda sounds like a noisegate shutting abruptly or something - It’s quite an annoying glitch.

The reason I think it’s a bug is when I send the audio to Mix2 via reaper and listen on the same speakers the glitch is gone. It’s not on the headphone outs either.

I’m using Reaper 6.15 64bit, Windows 10 - Buffer size doesn’t make a difference, Latency USB mode doesn’t make a difference. Using the UR midi input.

I can repo this by

Inserting NI Massive into a reaper project and play some notes with the default patch. (Also happens with some other soft synths, Sampletank 4 for instance, but not on all patches - but when it happens it’s only glitching Mix1!! - so weird)

I have an obvious work around which is to just use Mix2 for monitoring via the UR mixer, but really this is very puzzling. If it were one set of hardware outputs it’d make sense, but not ‘virtual’ ones! - seems like a mixer software problem?

Very happy to submit any further info but would like to know if anyone else has found this?

More info!

I had all the track inputs muted in the UR mixer on Mix1. If I unmute just one of the UR inputs the glitching stops. I can repo by muting 7 of them and then muting and unmuting the 8th.

Also if I leave all the inputs un-muted but turn all the faders down to infinity the glitching returns.

Looks like some audio path opening/closing when the inputs are muted? - I dunno, very easy to avoid this issue though.