Automatic Reply Email sent only once to the instance

Automatic Reply (out of the office) emails are not sent to the Instance all the time when any notification is sent to the user from the Instance. 

Automatic Reply is being sent only once from the outlook. This is expected behaviour as per Microsoft.

Check reference documentation:

https://docs.microsoft.com/en-us/exchange/troubleshoot/out-of-office-assistant/one-reply-sent-sender

Also, refer to KB's related to Automatic Reply(Out of Office) Issues: 

Automatic Reply Mails not recieved in the instance

'Out of Office' Auto Reply is not generated in some email clients on emails sent from an instance

Error in subject line of email sent from ServiceNow "[WARNING: UNSCANNABLE EXTRACTION FAILED]"