Opening a different contact's cases tab in a new browser tab while creating a new case causes new case to go missing

Go to a contact's cases tab.
Click on new case.
Fill out but don't save yet.
In a new browser tab visit a different contact's cases tab.
Go back to the new case you were creating and click save.
It takes you to some weirdo case search listing that says it's a list of cases for the other contact, but clearly isn't, and your newly entered case is nowhere to be found. There are no errors in the logs or on screen.