I'm having same issue after January patch on my A52 5G.
I'm having the same issue too on my S21fe since last software update. I went down to my "three store" where I bought the phone and they tried to give me some bull about it being part of the security update... but that makes no sense at all. There appears to be no specific idle time / inactivity period when it stops activating the sensor, can be a few minutes or few hours. I hope it is just a bug and they fix it soon. It just makes the whole thing feel clumsy having to double tap or hit the power button each time.