Are you using the new MLAT client,
From another thread
Please note that I have been advised as follows that a new MLAT client is available which should fix the previous issue
Package mlat-client is updated in our repository using the latest published version from mutability (0.2.11), but ours is "1:0.2.13" just because we need to change version number to 'force' apt-get to understand that is a new version (the code is the same).
Should work for every new installation by now.
Note that the packages are updated only for the latest (current) version of raspbian OS (AKA Bullseye), not for older versions (buster). If we have A LOT of users running older OS's, we can create a package for that.....but my recommendation is to update the operating system.
Alan