Holybro F9P RTK Bias

I’ve been using the Holybro F9P RTK base station without issue for the last few months, < 5 cm nav error using fixed landmarks on google earth. In the last few days, QGC has been reporting a nav bias of ~ 2 meters when RTK is connected and converged. I have reverted my flight software/parameters to a known working baseline but the bias persists. Any ideas?