This 400 error can happen if you have not configured a routing rule for the frontend host. A routing rule needs to be explicitly added for that frontend host. Refer to this.
You could get a deep understanding from How Front Door matches requests to a routing rule and check if a valid request path in the routing rule matches the frontend host.
If there isn't a custom forwarding path defined, then the request gets forwarded to the appropriate backend in the configured backend pool as is. Else, the request path is updated as per the custom forwarding path defined and then forward to the backend. You could verify if the forwarded path
is valid when you set the custom forwarding path
.
If you have any question, please let me know.
update
For configuring a Front Door for a static storage site in Azure. You could select a custom origin
using the static website's URL instead of blob storage origin. Moreover, if you want the URL to be rewritten using a regular expression. You could use Azure CDN for this static website instead of an azure front door. You could override HTTP behavior using the Azure CDN rules engine, so requires the use of the Azure CDN premium offering from Verizon.
More reference: Serving Your Static Sites with Azure Blob and CDN