Package: fbzx Version: 3.1.0-1 FBZX is a bit out of date. The current version in Buster is 3.1.0 which has issues with full-screen where it changes a multi-monitor setup to mirrored from what-ever you had previously. Also, it doesn't seem to append to TZX/TAP files when SAVEing as it should.
The upstream version 4.0.0 has neither of these issues.