Zwifty Hub Classic HRM Passthrough locking up

I’ve used the Zwift hub classic with the hrm Passthrough (with both and old Garmin and newer Garmin dual cheststrap HRM) for almost 2 years with no problems, but in both of my last 2 ZRL races the heart rate has locked while in the pen…
I’ve done other races and workouts since and it’s not been an issue, only the ZRL pens!
It shows a solid unchanging number, 90 last week, 81 this week, I can disconnect and reconnect it (through the Passthrough) and it will show the same number…
Totally messes up the start of a race if you’re faffing with tech stuff… The annoying part is the numbers are close enough to what they should be that it’s really hard to notice.
Anyone else had this issue? I’m lucky enough to be able to connect direct if I need too…