Add the -all switch:
Migrate to the PS MgGraph as well :)
[https://stackoverflow.com/questions/74307200/microsoft-graph-to-list-users-assigned-to-app](https://stackoverflow.com/questions/74307200/microsoft-graph-to-list-users-assigned-to-app)
$Assignees = Get-MgServicePrincipalAppRoleAssignedTo -ServicePrincipalId [objectIdServicePrincipal] -all