TP-Link load balancer keeps falling back to backup connection - even when primary is active
I'm using the TP-Link TL-R470T+ load balancer to manage automatic failover between my primary connection (ISP) and a backup cellular connection (also provided by a TP-Link router).
I have a simple backup rule configured: if primary fails, fail over to backup.
On several occasions, I have found the load balancer using the backup / failover (cellular) connection when the primary connection seems perfectly intact. Upon manually removing the backup connection from the load balancer, I can confirm that the primary is working just fine.
Is there any reason this might be happening? And a way to diagnose what the root issue may be?
Hardware: TL-R470T+ v6.0
Firmware: 6.0.4 Build 20200313 Rel.32850