I am seeing the same issue with my Pixel 3a. It started after the June update. My battery literally drains in about two hours from 100% with the screen off and no use of the phone. I have tried uninstalling as many apps as I can, leaving only the standard Google apps, all updated to their latest versions. Restarted phone several times. It still happens. I tried disabling adaptive connectivity, and the various other suggestions, including restricting battery usage for as many apps as I could. It still happens. I reinstalled my apps and looking at battery usage now, I see Carrier Services (7hrs3mins) and WhatsApp (1hr15min) at the top. However, since it was still happening with WhatsApp uninstalled, that does not seem to be the cause? I tried enabling developer options and disabling background processes. Still happens. And so on. I haven't found a solution so far. The phone is simply not usable now without a battery pack constantly attached to it while I'm travelling. Google - this is really bad. Please fix it.

Update 3rd August: a new version of Carrier Services was installed yesterday and the issue is still there. I am literally having to keep my phone plugged it at all times to keep it charged.

Update 4th August: I restarted the phone in safe mode, left it with the screen turned off and with no use, and the issue is still present - battery drains completely from 100% to 0% in about two hours.