Hi,
Had a look around the forums and it seems this is a longstanding issue (since 2017?!) - Tested it myself and am able to reproduce the issue - the workaround is to add a card in the normal way (+) and choose one of the existing fields, then unlock it and change it to whatever field you are intending to use.
Although, In your case (since I also use SharePoint as a back-end), I'm curious as to why you don't just add it using the normal field selector? You can unlock the card once added and it effectively becomes a custom card that you can do what you please to it,
Could you let me know if there's a specific reason for doing it this convoluted way of adding a custom card and setting it up from scratch - if the field already exists in SharePoint then just add it using the Field Selector - if the field doesnt show up as you've just added it to SharePoint then Save your app, close it compeltely, open it for editing again - this will then show you the new fields.
Cheers,
ManCat