I have a similar issue using safari on an iPhone but it also prevents me from posting more than once on a thread.
My latest user name is automatically in the the user name box but I get a message telling me I need to add a user name. I have to retype it to be able to post.
Although it lets me post once I have re typed my name, I still get the same error message even though it has posted, which has led to multiple duplicate posts before I realised…
I can’t post a second message on a thread because you can’t change your user name on a thread so the retyping the user name work around isn’t possible.