Hi @Gustav_Samuelsson and welcome to the Forums!
What you say makes sense. I contacted a higher technical authority on the Zwift support team, and as it was explained to me, Polar watches are meant to measure power/wattage when running, but can also display cycling power readings. As far as I’m aware, this will only function, however, when hooked up to Polar’s own compatible smart pedals, and won’t function with another device (e.g. Wahoo KICKR).
Perhaps the expectation for some is that it would get the power reading from Zwift or from their trainer when cycling, but in reality it might just be attempting to measure ‘running’ power, and this is resulting in inaccurate numbers.
Ultimately, it does seem like a compatibility issue.