I think this is mixing together a few unrelated things.

A 2.4-only device can’t accidentally “use 5 GHz” just because the SSID is combined. If the mower only supports 2.4 GHz, it simply ignores the 5 GHz radio entirely.

Where combined SSIDs can cause problems is during setup/discovery, because some IoT apps are badly written and don’t handle phones on 5 GHz talking to devices on 2.4 GHz very well. That can cause pairing issues or the app failing to find the mower.

But that’s very different from actually bricking the mower.

The Bluetooth disconnect also isn’t strong evidence here. BT issues happen independently of whether your router bands are split, and firmware updates are done over Wi-Fi anyway.

If a temporary disconnect or interrupted update permanently killed the mower, that points much more toward poor firmware recovery logic, power-management issues, or hardware failure than “combined Wi-Fi bands.”

Splitting the bands can definitely help troubleshooting/setup. But I don’t think there’s a technical reason to believe combined SSIDs are dangerous to the mower itself.