Hello @Tuan Tran,
welcome to this moderated Azure community forum.
If you use the the Azure portal to change devices registration, it can feel these changes are active without any delay.
Still, changes to an IoT Hub device registration in the device registry can take some time because these are handled in a asynchronous way.
This is especially seen when multiple devices are changes in bulk.
I recommend waiting a bit longer and perhaps eg. use the AZ CLI new to the portal UI, just to be sure.
Keep in mind the EventHub only deletes message once these messages are expired.
If the messages are consumed (by some consumer group) it feels these are deleted but the Event hub supports a replay of older messages.
Please check the timestamp of the messages.
If this is not helping, an Azure support ticket is recommended so the support team can look what is happening with your Iot Hub.
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