@Emitplink, this is a clear indicator that the WiFi coverage of the 2.4 GHz and 5 GHz bands is not equivalent, at least for the new iOS 14.

You could disable band steering, but if you do so it's recommended to also use different SSIDs for both bands, else roaming requests will still occur.

Band steering requires that the WiFi coverage of the 2.4 GHz and 5 GHz bands is equivalent. Since 5 GHz has a much larger attenuation compared to 2.4 GHz, the rule of thumb is to have a 6 dB (four times) stronger signal in the 5 GHz band than in the 2.4 GHz band if you want to use band steering. If there are walls or any other obstacles, you probably need more APs to achieve a better coverage.

As for new versions of iOS: Apple has a famous history of screwing up their WiFi with every new iOS version. Usually they fix it with updates later, so maybe it will become better with the next update.

Anyway, you should check WiFi coverage with a WiFi analyzer or a WiFi heat mapper.