Map New UPN's for On-Prem SSAS cube

Hi,

On the PowerBI Service we could previously map external users to an internal AD user. For example:

[email protected] --> [email protected]

[email protected] --> [email protected]

It seems that we are no longer able to map the UPN. This essentially is to allow external users to use the row level security on the on-prem tabular SSAS cube.

When we select the settings via the data gateway, the list of users stops half way down, and the Add New Rule button doesn't work:

Has anyone else experienced this issue?

Have microsoft limited the amount of entries? How do we fix this?

Any help would be appreciated.

Thanks