RST-ACK sent by the server after there is a FIN-ACK from the client

Hello

I have a few questions related to the type of connection that is continuous (longlive connection), some connections on the device experience RST-ACK sent by the server after there is a FIN-ACK from the client.

in transactions, it occurs like an intermittent transaction.
is this RST-ACK from the server the cause? or is the RST-ACK that is done after this FIN-ACK normal, because in transactions, the response from the request has been sent.

Thank you