VO₂ max : same number every time when using AW5 with external HRM (Polar H10)

Hardware:

Apple Watch 5 - watchOS 6.1.3

iPhone XS Max - iOS 13.3.1

Polar H10 chest HRM

tl;dr description of issue: VO₂ max recording for outdoor runs (always 20+ minutes) will not reliably calculate when using an external bluetooth HRM (Polar H10 for me). Instead, it logs the same VO₂ max value as was previously recorded. The recordings work fine if I don't use an external HR sensor.

I've been using an Apple Watch 5 for a few months using the integrated wrist-based HR sensor for my runs (always using Workouts app and with iPhone on me). After each run, a consistent VO₂ max value was added to my Health profile, each entry being different from the former.

Recently I purchased a Polar H10 chest HRM (on 3/9) for better HR fidelity and synced it to the AW (but never to iPhone). I can see my HR updated in real-time on the AW during my runs, so I know the chest sensor is properly transmitting via bluetooth to the watch. At the end of the run, my VO₂ max value calculation is always exactly the same as the last one. See screenshot below.