Apple has already announced that they will issue a software update to address this.
The iPhone 12 probably was compliant at release, then software updates increased the emissions for some reason and a retest has shown that it’s now above the EU legal limit of 4 watts per kilogram (~5.7 watts it seems). The update will presumably bring that down again, possibly reducing modem performance slightly.
This isn’t a real health threat as the legal limit is very conservative. That’s a good thing obviously, but iPhone 12 users don’t have to be worried for their health in this case :)