The reason why some of your requests may not have been successful is because of the rate limit, which limits you to 800 requests per minute, so if you’re making a significant number of requests quickly, then you may hit that limit. The only way to know if that’s your issue with some requests not going through is by you logging the response of your requests, as the error in the response would indicate what is wrong.
It’s not a bad thing, as resubbing has to be done if subscriptions die due to a restart, but ideally the sever used for EventSub should be stable and not need to be too frequently restarted. If your app requires 100% uptime of subscriptions then you would need a 2nd server that you can have subscriptions on too so that when one server is restarted the other still gets the notifications and none are missed.
If server downtime is a common thing for your situation, then perhaps EventSub isn’t the ideal solution for you and polling the API endpoints may be more suitable.
You can use the Get EventSub Subscriptions endpoint to check which subscriptions are active and which ones have been disabled due to your server being offline when notifications were attempted.