The issue happens for products which were inserted into database prior to Field Accessibility being added for these fields for the External User/Buyer Profile.
Changes to FLS do not invalidate Cache entries for Product2 records due to which the data will not be visible to External Buyer Users unless the old cache becomes invalid and new cache with updated data becomes effective.
Steps to Reproduce:
1) Add a new custom field to Product2 (Do not set FLS/Visible checkbox for the external Buyer User's Profile)
2) Add the new custom field on the Product Detail page in Builder
3) Publish changes in Builder
4) If field requires data, populate data for some number of Product records. (Now Establish field-level security for the field to Buyer User)
5) Login to the Store as a Buyer User
6) Access a Product Detail page for a Product with data populated for the custom field
7) As System Admin, set Field Level Security for to Visible for the Buyer Profile
8) As the Buyer User, reload the same Product Detail page
You can follow either of the following workarounds. If neither approach is taken, the issue will resolved when the bad cache entry eventually becomes invalid(30 days after it was first added to database).
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