I get only one bar on my 12 pro max at places where my 11 pro max got two. My wife has my old 11 pro max and continuously gets better service than me. It's odd.
The bars aren’t an accurate representation of signal strength. There’s a way to actually check signal strength but I forgot how. I came from an XS Max and it’s the same. I never get dropped calls and I have signal everywhere. If you’re having issues maybe you have a defective iPhone. Do you have a thick case?