Turning the volume to max on iPhone doesn't resolve the problem. No matter I change the volume after in the headphones, the beep is still there between songs.
Really I think it should be a problem with Bluetooth profiles or something like this. I have Xiaomi Mi Sports Bluetooth that has an independent volume control and it does not beep. Bose SoundLink controls iPhone volume and it does not beep.
But SoundPeats Q9A and Sennheiser HD 4.50BTNC controls iPhone volume and they are beeping. Maybe it is a problem with aptX codec, AD2P profile, BT version or whatever... iOS 12.0.1 launched yesterday didn't solved the issue.
Anyway best solution is Apple to fix it. We don't want a confirmation that I've just pushed one button, I know what I'm doing in my headphones.