Export Custom Fields

Hi everyone,I’m with a little issue and confuse, I want to export a custom field so I add the new field in setup -> customize form, then I’ve choosed the doctype. Ok everything was fine until I migrate the change. When I give the command “bench migrate” my field was just added in the table “tabCustom Field” not in the doctype. I guess that with images will be more easy to understand.

First image is of the doctype Purchase Invoice Item:

Second image is after I clicked on the button to export customizations that was created my folder “custom” and inside the .json file:

itemjson

And now is the issue in the database was create field just in the "tabCustom Field:

And that’s the table of the doctype

tabPurchaseItem

The weird part is when I export a field using fixtures and give the command “bench export-fixtures” works fine,
the field is added in the both tables but with the right way happens this.

the field “Landed cost voucher” was added by 'bench export-fixtures" and works fine. and the field “Stock Entry” with the right way and doens’t work.

Thanks for advance