APIM Error: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host

We are getting the following error, Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host
Our Backend is hosted on AKS and is behind nginx. When we try to process a file which takes more than 5 minutes it throws out this error, we checked on nginx level and always got response. But when went through APIM it errors out and gives below error message.

We have increased the timeouts as well, also find the policy xml below.
<policies>

<inbound>

<base/>

</inbound>

<backend>

<retry condition="@(context.Response.StatusCode == 0 || context.Response.StatusCode >= 500)" count="10" interval="10" max-interval="100" delta="10" first-fast-retry="false">

<forward-request timeout="1200" buffer-response="false" buffer-request-body="true"/>

</retry>

</backend>

<outbound>

<base/>

</outbound>

<on-error>

<base/>

</on-error>

</policies>