F5 Trunk port is connected to Cisco Switch Access Port

I'm not an expert on Cisco, but in my opinion, portfast has no business in the datacenter, and definitely not on a switch that would be connected to servers or other network devices. Portfast is meant for stuff like desktop PCs and other end user devices.

If the F5 ports are configured as trunk ports with a VLAN tagged and they end up connected to an access port on a switch, I'm fairly certain it would not pass traffic.

The private connection between your F5 units should never be configured to allow traffic to enter via the switch and then cross that link, so should not be able to create a loop. (ie: switch -> f5 1 -> f5 2 -> switch, should never be possible unless you configure it to be so, and is a bad practice if you do configure it as such.) That connection should be setup to only allow each f5 to contact it's peer.

The scenario you are describing would require mis-configuration of the F5, the switch, and the cabling, and is very remote.