@me @robyn Yup was able to track down the docs page. This is the case
> MUI functions as a client, similar to the Android and Apple apps. It runs on one core of the MCU, while the regular Meshtastic firmware runs on the other. MUI communicates with the firmware using the Client API, the same interface used by other Meshtastic clients.
> The Client API supports only one active connection at a time, which means other clients cannot be used while MUI is running. Since not all settings can be configured within MUI, you may wish to boot into Bluetooth Programming Mode or BaseUI. This frees up the API so client apps can use the Bluetooth or serial connection.
https://meshtastic.org/docs/configuration/device-uis/meshtasticui/