Hi,
It's the first time I have seen this issue, but it strikes me as, by design. Because you are using the sections function in the DFW, the software might not know which section you want to add the new rule too. I wouldn't be surprised if this is "fixed" in a later version of the code. What version are you using?
Try adding a new rule to a different section using the same UDP-8888 service. Filter by service 8888 and filter again, you should see 2 rules in 2 different sections (hopefully). Does that then allow you to add/delete rules?