Bold of you to say the car doesn‘t matter, because my Mercedes updates the signal strength over bluetooth just fine. And yes, my phone is in standby in my pocket.If you have an Xs or Xs Max, it doesn’t work regardless of what car you have. I’ve personally got a Mustang with Sync 2 and a Grand Cherokee with the updated uConnect, but have also tested with Sync 3, older uConnect, and MyLink systems as well as with multiple devices.
CarPlay will update signal strength as long as the phone is doing something (streaming music, navigation, etc), but Bluetooth will not update signal strength if the phones display has been turned off for more than a few seconds. Upon turning the display back on, it takes about 5 seconds for the signal bars to update on the phone, and be reflected on the in-car display.
The issue isn’t with Bluetooth, it’s with the phone not updating the displayed signal level in the background while the phones display is in standby mode. To replicate the issue, go to a strong signal area, turn your phone display off and keep it off, move some distance to an area you know has a weaker signal and turn your phone display back on. You will see full strength immediately upon turning the display on, but after a couple of seconds it will fall to reflect actual signal at the current time.
Once again, Xs and Xs Max only AFIAK. Might impact Xr as well, but haven’t had one to test. Does not affect X or any older device.