Negative output voltage on telemetry line of two Herelink air units

We recently bought a Herelink unit and connected it to our Cubepilot Orange. All seemed fine for a while until one day it simply stopped working. We figured it had gone dodgy, so we replaced the Cubepilot but the problem persisted so we replaced the Herelink unit too.

A voltmeter gives no continuity from the telemetry ground line to any other grounds. Putting an oscilloscope across the Herelink telemtry line shows that relative to the antenna and USB backshell grounds the telemetry line is pulling negative 3.3V, which is very surprising. Both Herelinks are doing it, so I wonder if it’s something common to their batch.

Pairing and SBUS seem to work fine.

Any ideas on how to fix it, or if we just have dodgy units?