I duplicated something like it in 2.9.4. I created a 16 character nickname for V1, but also had V1:5 used in my program, so there was an implied nickname for that with V1's nickname and ":5" appended to the nickname field from V1.
I then edited V1:5 Extra Info field and the nickname for it CHANGED from "HtrPB_BitsFrom_1:5" to just HtrPB_BitsFrom_1 (matching the NN for V1).
I tried it with my development of 2.10 and it's behaved differently. The V1:5 implied nickname did NOT show up, but when I edited V1:5 Extra Info, V1:5 Nickname DID become V1's nickname with :5 appended, BUT ALSO the Extra Info changed from what I typed to "5".
We'll figure it out.