SOLVED - I figured out my problem. It was caused by this setting below (Static ARP under the DHCP Server configuration for the interface), which I had enabled on the interface because I interpreted it incorrectly. It essentially took precedence over any and all allow rules configured for the OPT2 interface, and prevented any host without a statically assigned DHCP address from communicating with the interface even though the host received the dynamic DHCP assignment from the OPT2 interface. I hope this saves other folks time and headache.
Screen Shot 2019-11-06 at 9.46.34 PM.png
As explained in docs.netgate[.]comScreen Shot 2019-11-06 at 10.40.04 PM.png