Can't login to make a donation from business account secondary user account

[patjammer]

I've created a secondary user account on our business PayPal account.

The secondary user can login as normal from https://paypal.com but they are unable to login to complete a donation from a paypal.com/donate link

The primary account (mine) has no problem signing in with the same PayPal donate link.  Same browser, same device.

The secondary user account has both an email and a username that is not in the form of an email (no special characters allowed).  From the logon prompt for the donate link, it does not accept the username.  It results in an error message "that email or mobile number format isn't right", which makes me wonder if it is some deprecated api method or something in play as it doesn't even seem to accept anything like the secondary user's username which is not an email or mobile number.

Any clues or things to look for?

[patjammer]

For the benefit of anyone who also has this problem and finds this in the future, PayPal customer service informed me that secondary user accounts may not log in to any flow for "make a payment".  Can't authenticate to make a normal purchase, and can't authenticate to make a donation.  The only thing you can do with it is log into paypal.com and manage things from there.  Can't login from a payment button/link from another merchant.  You are welcome for the hour I put into this that you don't have to now to understand this undocumented limitation of pretty much the only reason I even created a secondary user acccount.

Lame.