So the output should be correct unless the binaries you are running don’t match the ones on-disk, i.e you will need to reboot and reproduce again to get a complete output with coredumpctl gdb.
After upgrades, it is important to reboot at some point.
You might have doesn't match build-id from core-file during file-backed mapping processing message in the command output indicating it.
Btw the result of the command is stored in the file backtrace.txt when using coredumpctl debug plasmashell -A "-ex bt -ex quit" |& tee backtrace.txt.