I use the phone bluetooth mostly in my car for music and phone calls and with a bluetooth speaker when I'm on the lake on my Stand Up Paddle board. With my iphone 6s, it worked flawlessly, never had an issue.
I upgraded to an iphone 11 pro last year, and since then, audio keeps stuttering in my car (music and phone calls), and with my bluetooth speaker as well.
The only solution that works 100% for me is to have the phone away from the car radio or the bluetooth speaker. For instance, if I put the phone in the car door instead of the "normal" spot under the car radio system in the center, I don't have any issues with music or phone calls. As soon as I bring the phone closer, I start to have issues. Same with my UE boom 2 bluetooth speaker, if the phone is just next to it, it stutters and cuts the music, but if I take the phone away from it (1-2 meters), I have absolutely no issues anymore.
At least I found a way around the issue, but I'm still really annoyed with it. When you pay so much money for a supposedly premium product, you expect it to work properly...