There is a use case where the user chose a particular domain "DomainA". Then he is sent to IDP.

At the IDP, the user does not do anything or logs incorrectly.

The user then goes to the SP url directly. He is shown the account chooser page.
But when he chooses his domain, the IDP he is sent to is the fallback idp set in the identityurl of the SP - not the IDP for the domain he chose.