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?
I've just checked on my version of 6.4.0 and I have the same issue.
My version 126.96.36.19943711
A very strange situation. Managing rules with a large number of sections is not very convenient.
It can be a little messy, but it's the same issue when you have 100's of FW rules regardless of sections.
Try minimizing all of the sections and only opening the one you need to edit. I find that helps. Continue to use the filter to find rules you need to check or edit.
Unfortunately what you've described has been around since the very beginning of NSX (i.e. 6.0 days), you just learn to workaround/live with it.