How to check backup against SQL AG by Get-DbaLastBackup?
Get-DbaLastBackup currently doesn't support the ability to check the last backup on a database that's in an availability group.  You will either need to modify the GetDbaLastBackup code or use another script that queries the servers in the availability group, collects the last backup information from each of them, and returns the latest date that was collected.

This would certainly be a nice feature--even some of the big money SQL Server monitoring tools don't provide a nice mechanism for monitoring transaction log backups on availability groups.