Regarding the Event subscription failure

Hello, 
In our current system, we are using the Qualtrics event subscription API to trigger our custom logic. So while the event subscription creation, we get the success response as mentioned below: Reference_1
And according to this success response, Qualtrics has started to call our domain when the new response we get as mentioned in the below request event: Reference_2
But for the last four days, It has stopped working. According to event subscriptions retrieval API, it is showing the "subscription id not found" (Reference_3). According to Qualtrics API documentation, we can't delete the event subscription from the Survey. So how our event subscription got deleted? Please explain the necessary scenarios in which these things can happen !!
Reference_1 : { "result": { "id": "SUB_dijybzr2xxIrfrU" }, "meta": { "httpStatus": "200 - OK", "requestId": "e63e6e59-25f1-47b1-aed0-4f5b3e3569ab" } }
Reference_2 : {'survey_data': 'Topic=purpics.surveyengine.completedResponse.SV_9QqF5LilbWsEzgq&Status=Complete&SurveyID=SV_9QqF5LilbWsEzgq&RecipientID=&ResponseEventContext=&ResponseID=R_1ozn6VQAj3Cden9&CompletedDate=2021-03-05+02%3A36%3A52&CreatorID=UR_5hTRTI29c9IehNj&DistributionChannel=anonymous&Duration=295&Language=EN&SurveyVersionID=9223370422053822801&IsPreview=false&IsTest=false&HasTextEntryAnswer=true&LinkType=Anonymous&BrandID=purpics'} 
Reference_3: {"meta": {"httpStatus": "404 - Not Found","error": {  "errorMessage": "Subscription Id not found","errorCode": "ESSG_4.0"},    "requestId": "c7c17ef9-daf7-415f-a144-de8848580645"  }}