Not sure if this is helpful, but I have had this issue for well over a year on just one of my DEV sites. For others it hasn’t been a problem, but then again they aren’t as big or the main one that I work on.
My fix to get my debug.log file to populate incoming errors was to simply delete everything in the app/logs/php/error.log file. Needless to say over a long period of time/coding and debugging that file was immensely full. So as soon as I cleared it my debug.log started to populate again. Hope this helps someone.
Was this issue ever resolved? I’m having the same problem with Local v 5.2.5+2498 and WordPress 5.3.2. The logs are being written to but my debugging error_log lines are not showing up in the logs.
Make sure to clear your php/error.log and then check to see if anything outputs to it. That’s what fixed it for me. There was just too much in there for it to want to update it.
Are there any errors on the front-end of the site? It’s possible that the error is being triggered early enough in the process that WordPress doesn’t have a chance to catch it and log it.