device keeps being found and removed

I've got this problem on a bq ubuntuphone.
I've used a bluetooth keyboard with it several times. Today I pressed
a button "forget this device" on the UI of the phone. Afterwards I
cannot see the keyboard again when a scan is performed.
I connected to the phone with ssh and found that a CLI tool was
present: bluetoothctl.

From this tool, when I press the pairing button on the bluetooth

keyboard I see a message with NEW followed immediately by a message
with CHG and one with DEL referring to the keyboard.

What's the problem?

TIA. Best regards.

--Luca