VO2 max with external heart rate monitor (chest strap)

Hi!

I used to have an AppleWatch 4, and used it to go running with a Polar H10 bluetooth heart rate chest strap. I used to get VO2 max readings after every run.

I bought the new AppleWatch 6, and use it to go running with the same Polar HR chest strap, but never get a VO2 max. If I don't use the Polar HR strap and rely on the HR the Watch itself measures, I do get VO2 max readings.

Has anybody experienced the same issue?

Thanks!