what is the MAC address of a floating IP Address in f5 11.3.0 TMOS ADVERTISES TO NEXT HOP

Are you using Mac Masquerade? If so, then whatever is set, will be the mac advertised. If not, it would be the lowest-numbered interface associated with the vlan.

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos_management_guide_10_1/tmos_vlans.html