There is a workaround that will set the correct key signature everywhere but on the LCD at the top. Just don't use that LCD setting, Custom is much more useful.
the workaround is to first go to Logic Pro XPreferences/ Advanced Tools and make sure that Show Advanced Tools and Advanced Editing are both on.Once these are on you can elect the Custom LCD setting too.
Go to the top right and click on the List Editors icon. Then select the Signature Tab below it. This will show you the current Key signature and you can change it to what you want. If you tried to change it to BbMaj and the LCD says EbMin, your Key Signature in the Editor may already be correct.
The only place that the Key Signature is incorrect is in the LCD. Your notation will have the correct Key Signature.
Rick