A very recent thread here LIRC not working on Pi3 after October 2020 update - #15 by andybadshot contains a suggestion that this might be an issue with Debian Buster and/or kernel 4.19.
There is another thread on the Raspberry Pi forum, dating as far back as March 2019, where this issue is discussed: [Bullseye/Buster/Stretch] Using LIRC on gpio-ir with kernel 4.19 or later - Raspberry Pi Forums