Yes, it’d be great if this one can be put back up. The fix you shared by @jordan-vidrinehere adds the login to the bottom of the page but not to the header. It is important to have the Login where it usually is because it’s more visible, please.
I definitely understand that perspective, but the reason it is hidden on the invite redemption page is that “Login” in the header could be confusing as the first CTA they see, before they begin the account creation process.
If you want the button to be visible, I’d suggest un-hiding it with custom css in a theme component on your forum.
This takes into account the users who are ‘new’ and don’t already have an account, but what about those who already have an account but were not logged in when they used the invitation link. It would be confusing to see a long form that they are ‘requested’ to fill in ‘and create a new account’ when they already have one (without a clear indication that if they already have an account, all they need to do, is to login.)