Dynatrace Managed has been implemented for a few months on one of our websites. We recently noticed that problems stopped being raised for failed requests even though there's a 100% failure rate. Did Dynatrace see this as "normal" through automatic baselining and therefore stop raising problems for the failed requests? If so, how can Dynatrace be configured to always alert on 100% failed request for a particular transaction? I've tried setting it as a "key request", but still have no problems raised for the request.
Some others may be able to add something but personally I would open a support case for this so that they can investigate a bit further. One thing I might check though is that you don't have any custom settings for anomaly detection that could be impacting this.