Please review below points:

1. How do you subscribe HTTPS endpoints to your SNS topics? (Amazon SNS console [1], the sns-subscribe command [2], or the Subscribe API action [3]) Do you see any errors during this process?
2. Could you please confirm the HTTPS endpoints have the capability to handle the HTTP POST requests that Amazon SNS uses to send the subscription confirmation and notification messages [4]?
3. In addition, as the endpoint type is HTTPS, the endpoint owner must run the ConfirmSubscription action to confirm the subscription. You need to call the ConfirmSubscription action with the token from the subscription response [5].
4. Could you please try the sns-subscribe command [2] with verbose mode?
5. AWS SNS sends the messages with “Content-Type:text/plain; charset=UTF-8”, and if the endpoint isn’t configured to accept content type same as SNS it will not receive the confirmation messages. Could you do a test using a curl command to an API Gateway endpoint to check whether this is supported?

References:

[1] Step 2: Subscribe the HTTP/HTTPS endpoint to the Amazon SNS topic – https://docs.aws.amazon.com/sns/latest/dg/SendMessageToHttp.subscribe.html

[2] https://docs.aws.amazon.com/cli/latest/reference/sns/subscribe.html

[3] Subscribe – https://docs.aws.amazon.com/sns/latest/api/API_Subscribe.html

[4] Step 1: Make sure your endpoint is ready to process Amazon SNS messages – https://docs.aws.amazon.com/sns/latest/dg/SendMessageToHttp.prepare.html

[5] Subscribe – https://docs.aws.amazon.com/sns/latest/api/API_Subscribe.html