
The native SharePoint Create Item connector in Copilot Studio simply does not support multi-select choice columns reliably. Every format you tried was logically correct, but the connector cannot serialise multi-value table input properly when sending to SharePoint, which is why you keep hitting the 400 error.
The Power Automate route is the right fix for now. Pass your values into a flow, use a Select action to format the array correctly, then write to SharePoint from there. It is an extra step but it is the only reliable path until Microsoft closes this gap in the native connector.
Worth raising it on the Power Platform Ideas board — it helps Microsoft prioritise the fix.