FR235 not handling HR (OHR and External) properly since 6.10 ?

Early FR235 adopter here and after some teething problems I'd found the OHR to work reasonably well (even on longer intervals) until v6.10 came along and messed things up badly. Ok so it's also the time when the weather got colder so maybe the OHR was at "fault" but ever since, regardless of the tactics employed (rebooting before running, rebooting during a run, etc...) HR has been mostly unreliable.

After trying all kinds of combinations of straps (Fabric v1, Polar, new OEM Polar) and pods (HRM3, Viiiiva, Geonaute) I broke down and just got a brand new "Premium Heart Rate Monitor" (the HRM3-SS) with the Fabric v2 strap and guess what...I still got ridiculous numbers after starting my run (should have been around 120, it just went up gradually to 170) until I rebooted my FR235 after about 5'. Then (like with the OHR) it settled and gave me realistic numbers :



At the end of my outing I did 4x short sprints and the HR barely budged :




It's really hard not to think that there is something "off" in the way the FR235 handles HR. Yes I know, the common wisdom is that the strap "does it's thing" and the watch just uses whatever data it gets but there has to be something at play here. I'll dig out my FR610 next week to try to get to the bottom of this. I thought that if the HRM3-SS still didn't cut it I might get the HRM4-RUN (the red one) but it seems like it would be a waste of time and funds...