Since yesterday, the username field populates with my registration email.
Trying to be helpful:
I think my password manager may be pre-filling it, but it's only started in the last couple of days. The password manager triggers on www.mumsnet.com/login though, so the glitch might not be coming from my end at all.
I don't mind trying a couple of experiments, but I'll have to keep logging out & in again to do them, so posting this first!