I contacted the Partner support but they referred me to this forum.
I verified that the development store has the app/uninstalled webhook installed via endpoint '/admin/api/2020-07/webhooks.json'. However, when the app is uninstalled, the app/uninstalled webhook is not fired at all. Our server doesn't receive any information. There is no log in the Partner dashboard that we can check if the webhook is actually fired or not.
Can you provide the name of your app, and also the shop_id (or DM me the shop name) where the webhook didn't fire. I'll check our logs for more information on the webhook.