Did you solve it? I'm currently logging in with a Japanese Apple ID, and my Store account is also from Japan, but ExternalPurchaseCustomLink.isEligible is still returning false. I've configured it with:
.entitlements:
com.apple.developer.storekit.custom-purchase-link.allowed-regions
jp
SKExternalPurchaseCustomLinkRegions
jp
Topic:
App & System Services
SubTopic:
StoreKit
Tags: