Magento 2 Product Attributes not appearing

My Magento 2 store has a couple of different types of products (E.G Simple, Configurable, Grouped, Bundle, Custom, etc). For most of them, all the attributes show up correctly. However only for Custom Products, 2 important attributes (Tax Class and Restricted Countries) are not appearing when i create new products.

Custom Products is a type of product that is created by a plug-in i am running. The Restricted Countries attribute is a non-standard attribute that was also customised for my store.

These attributes are already specified to an Attribute Set that is used by Custom Products.

Any ideas why this happens, and how can i fix this issue to have these attributes appear?

submitted by /u/coreqwerty
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