I am having this issue. I have an iPhone 7 and recently updated to the new 13.4.2 iOS. It was hot this morning and after I turned it off to cool down, it appeared to resolve itself however now it is the evening and I am having the same issue again.
I am also having issues with the battery draining, scrolling glitches, issues with the camera loading to forward facing. I can’t engage Siri, it takes forever for the call to connect and then when it appears to have connected I can’t hear anything. Someone tried to call me and it didn’t appear that I picked up on the other end but I had picked up the call but couldn’t hear anything. Speakerphone is grey’d out. I get no error messages when trying to make a voice memo, it just doesn’t do anything when I press the record button. I had issues with connecting to any speaker (google home or Sonos) yesterday via Bluetooth and had lagging and play issues on the Spotify app. While using Instagram my phone overheats.
These issues appear to come and go over the days, it’s not a constant issue but it’s a relatively new host of issues.