Running cfn-nag 0.8.10, scanning a nested CloudFormation template, outputting json and am seeing:

Experimental SPCM rule is failing. Please report undefined method `gsub' for {"Ref"=>"RDSKMSKeyAlias"}:Hash

      value = value.gsub("${#{special_character}}", '')
                   ^^^^^ with the violating template

[ ... rest of json output ... ]

While the CloudFormation template(s) may be erroneous, we would not expect plaintext errors to make their way into JSON output as this output gets scanned later for test reporting in GitLab.