This is only a temporary fix.Auto-unlock by phone just happened to me this week.
iPhone 11 recently updated to iOS 15.1 (was previously 14.X)
Model 3 with Version 2021.40.6
After the auto-unlock feature using my phone stopped working, I found that in the Tesla app, the section that shows Security normally showed my iPhone name, but after the iOS update it did not. I logged out of my Tesla app on the phone, then logged back into the Tesla app. I forgot to get a screen shot of it, but on the Tesla app, it would prompt me to register my phone key, just above the Controls section of the Tesla app. This would require my key card.
I followed the prompts to start the process in the app, placed my key card below the cup holders, then after the phone updated the app, my iPhone name now appears in the section that shows Security.
Tested out the auto-unlock and auto-lock a few times and it seems to be working normally now. Hope this helps others out.