Metronome creates funky notes on the score

Hi all, apologies if this has already been dealt with and I haven't been able to find it.

I'm using Musescore 4.1 on Linux Mint 21.1, connected to my Casio CTK-800 keyboard via USB. Two-way MIDI connection works fine.

When I use real-time mode (either foot pedal or metronome), the metronome "clicks" are appearing on my score as a group of four very low notes (G1, E1 and D1). The "clicks" come through on my keyboard as a mid-range tone; they sound like "clicks" on the computer speakers.

I've tried disabling MIDI audio output, but that doesn't change anything; I've tried muting the Metronome channel on the mixer, but it still comes through on the keyboard (and then into the score). It's almost like the MIDI outputs notes to the keyboard which are then echoed into Musescore and onto the score.

A shot of the score with the underlying "click" notes is attached.

Does anyone have any thoughts of what I might be able to try for this one?

Thanks so much for your help!