I did a Zigbee heal, and also unregistered and registered the dimmer again. It seems to be okay now.
Weird though.
EDIT: As I said, it all seems stable after a Zigbee heal, and re-registering the dimmer. I’m still curious as to what actually happened though - how did one switch magically link itself to 3 sockets in the blink of an eye? Any Zigbee experts out there who want to try to explain, I’m all ears.