@StevenT Normally in HA you can reserve mgmt interface to be dedicated only to management. This means that it is only reserved for that purpose. In HA, routing table is handled by primary node. Secondary node has no active routing table and for this purpose, you have to configure mgmt port on HA settings and add a default route so that port can use different path than normal traffic and not depending on normal FGT routing table.
In Standalone FGT this issue is not present, so you dont have to create an isolated routing table for Mgmt interface.
For your issue, can you please check if tacacs is reachable from MGMT interface? do a simple ping with source option and see what is happening. Do you have routing in place?
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