Power Automate – Export PBI Thin Report (Shared Semantic Model) with Dynamic RLS- Supported or Not ?

1) The Export to File for Reports API fully supports thin reports connected to shared Power BI semantic models.

2) The limitation applies only to external Analysis Services connections — Azure Analysis Services and on-premises SSAS via live connection. A thin report pointing to a Power BI semantic model within the same tenant is treated as a native Power BI connection, not an external live connection, and is fully supported by the export API.

3) Yes, for dynamic RLS based on USERPRINCIPALNAME() this is the correct and only supported pattern. The API accepts an effectiveIdentity parameter where you pass the target user's UPN. The export engine then evaluates RLS as that user and returns only their permitted data. But you need to parameterize UPN otherwise it would use your own credential which probably would break RLS configuration.

Alternatively, you could use dynamic subscriptions instead of power automate which is probably the easiest way to do. 

https://learn.microsoft.com/en-us/power-bi/collaborate-share/power-bi-dynamic-report-subscriptions