SubscriptionExceededMaxDomainLimit when purchasing App Service Domain on a pay-as-you-go (Microsoft Azure Plan) subscription

I'm trying to purchase an App Service Domain from the Azure portal, but the deployment fails with:

Error code: SubscriptionExceededMaxDomainLimit

Message: "The subscription is not eligible to create an App Service Domain."

Tracking ID: 857bbf0f-2934-4761-bbf4-b4ea6ea3e71d

My subscription is a regular pay-as-you-go subscription (Microsoft Azure Plan / Microsoft Customer Agreement), not a free trial or credit-based subscription, and it has zero existing App Service Domains. According to the FAQ in "Buy and manage an App Service domain", the domain limit depends on the subscription type, but a pay-as-you-go subscription should be able to purchase domains.

I contacted Azure billing support, and they told me a technical investigation would require a Standard support plan, which is disproportionate for an $11.99/year domain purchase.

Could someone from the App Service team check why this subscription is blocked from purchasing domains and lift the restriction? I can share the subscription ID privately if needed.