Yeah, this seems to happen because the ATC doesn’t seem to tune their own altimeter to STD altimeter, it keeps harrassing you with the altitude because they’re reading using the local altimeter under the transition altitude.
The workaround for this is that if they tell you you’re 300 ft above the assigned altitude, you disengage the STD altimeter and set it so that it’s showing you are 300 feet above your assigned altitude, then use your autopilot to descend back to that assigned altitude, that should silence ATC.
The problem with this is that as you fly through your flight plan, you’ll go through different blocks of weather, if these blocks have a different altimeter reading, ATC will start nagging you again, and you need to reset the altimeter again to silence them.
It’s a hassle right now, so another workaround is to use a custom weather preset, where you have a constant barometric pressure throughout the entire flight, so you don’t need to keep changing it.