Fedup
Thanks for the reply. I can reproduce the same issue that you see when signing into eBay's website in Brave on my iPad. This behaviour isn't ideal and AutoFill is specifically handled by iOS so 1Password is unable to control which fields get filled when you tap on an item since 1Password only provides the data for the experience.
I recommend that you report the issue to eBay so that they can see why their website isn't working properly with iOS AutoFill in Brave.
For the time being, filling does work without issue in Safari so I recommend either using that browser for eBay or copying and pasting your username into Brave (as you mentioned doing).
-Dave