It's due to the system bluetooth.service unit using ProtectSystem=full, commenting that line from the service and doing
$ sudo systemctl stop blueetooth.service
$ sudo systemctl daemon-reload
$ sudo systemctl start blueetooth.service
and sending the sig11 makes apport trigger
Note that systemd-coredump doesn't have this problem and register a backtrace in the journal even with ProtectSystem in use, which suggests it's an apport limitation
It's due to the system bluetooth.service unit using ProtectSystem=full, commenting that line from the service and doing
$ sudo systemctl stop blueetooth.service
$ sudo systemctl daemon-reload
$ sudo systemctl start blueetooth.service
and sending the sig11 makes apport trigger
Note that systemd-coredump doesn't have this problem and register a backtrace in the journal even with ProtectSystem in use, which suggests it's an apport limitation