I found this AWS document on mitigation of false positives in WAF (1) and how to override rules in the group to identify which is causing the issue. I see 6 rules in the Windows Operating System managed rule in my own account. You can set individual rules to "count" one at a time to identify which may be preventing your SNS topic from posting. See the documentation link for details.
If you can identify the rule causing the issue and don't mind leaving it in "count", you can go that route. Otherwise, if you have a CloudFront request Id (x-amz-cf-id), this can be investigated further via a support case.
** RESOURCES **
- AWS Managed Rules for AWS WAF - https://docs.aws.amazon.com/waf/latest/developerguide/aws-managed-rule-groups.html
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