MSSQL Health Monitor Issue

We're currently using the F5 to load balance across read-only shards for a specific feature within our infrastructure. I have had success getting a MSSQL monitor to work initially, but every time the backend servers are rebooted, the monitor stops functioning. If I make enough cosmetic changes to the health monitor, it eventually works again.

 

What's odd is if I connect via management studio to the backend nodes and run the exact query in the monitor (with same credentials), the receive string is what is expected. If I change the pool's monitor to a basic TCP connect, and then connect via management studio to the VS's IP, I can execute the health monitor successfully. But if I set the actual monitor on the pool, it's still not functional.

 

The monitor is doing a simple select to a function that returns a single row/column with a 0 or 1 to signify whether the service is available for use.

 

Any thoughts on what might be the issue here?

 

F5 Software: BIG-IP 11.4.0 Build 2425.0 Hotfix HF4 F5 Platform: BIG-IP 5000