A cloud-based identity and access management service for securing user authentication and resource access
Since Graph is showing isMultipleSelectionAllowed as true but My Access still only lets you select one answer, I'd lean toward this being a limitation with the My Access portal rather than something wrong with your API request.
I'd try creating a brand-new access package policy with the same setting just to rule out an issue with the existing policy. You could also test it in another browser, but I wouldn't spend too much time troubleshooting the browser if the behavior is consistent.
The important thing here is that Graph accepting a property doesn't necessarily mean the portal fully supports that functionality. I'd open a Microsoft support case to confirm whether multi-select is actually supported in My Access.
If it isn't, your best workaround would probably be separate questions or a custom request form using Power Apps and Logic Apps. That would give you control over multiple selections and let you process each answer individually without depending on the My Access interface.