When deleting an OPT interface that is not the last OPT interface, the firewall rules, NAT rules, DHCP Server configuration, and possibly other items get shifted to an incorrect interface.
Example:
1) Setup 4 interfaces, LAN, WAN, OPT1, OPT2
2) Add firewall rules on OPT1 and OPT2, putting the interface name into the description so you can see what happens.
3) Delete the OPT1 interface
4) Look at the rules on the formerly OPT2, now OPT1 interface.
The old OPT2 interface retains its IP configuration, but loses its NAT, firewall rules, etc. and has the previous OPT1 interface's configuration for those items. What it should do is shift the firewall, NAT, and DHCP Server configuration to the appropriate interface, and remove any configuration for those items for the deleted interface.
Hope that made sense.