Running BTBench between Android Device and UbuntuHost

I am trying to run bumble-bench between an ubuntu host and an Android Device (using the BTBench app)

What I'm doing,

### Android device
```
# Build and install BTBench
# Start in RFCOMM server mode
adb shell am start -n com.github.google.bumble.btbench/.MainActivity --es autostart rfcomm-server
```

### Ubuntu Host
```
# $USB is the BT transport
# D4:3A:2C:53:29:BC is the BT address of the Android device that I got from the device's About page
python3 apps/bench.py --mode rfcomm-client --packet-count 100 central --peripheral D4:3A:2C:53:29:BC/P $USB
```

#### Output
```
$ python3 apps/bench.py --mode rfcomm-client --packet-count 100 central --peripheral D4:3A:2C:53:29:BC/P $USB
INFO:root:>>> Connecting to HCI...
INFO:root:>>> Connected
INFO:root:### Connecting to D4:3A:2C:53:29:BC/P...
INFO:root:### Connected
INFO:root:@@@ Connection: MTU=23
WARNING:bumble.host:--- Ignoring event HCI_CONNECTION_PACKET_TYPE_CHANGED_EVENT:
status: HCI_SUCCESS
connection_handle: 50
packet_type: 52248
WARNING:bumble.host:--- Ignoring event HCI_CONNECTION_PACKET_TYPE_CHANGED_EVENT:
status: HCI_SUCCESS
connection_handle: 50
packet_type: 52248
INFO:root:*** Starting RFCOMM client...
INFO:root:!!! Disconnection: reason=5
```

I'm not entirely sure what "reason=5" is.

Any idea what I maybe doing wrong?