assignment in map by named index causes casting error

If I assign a value to a collection via a named index (key name) I get a casting error like so:

Failed to convert property value of type java.util.LinkedHashMap to required type java.util.Map for property methodConfigNames

Here is the code in question. This doesn't happen in Groovy 2.4.15. Because Grails 4 is running 2.5.6, many plugins are failing with this error.

void setMethodConfigNames(Map<String, Map<String, List<String>>> methodConfigNames) {
    methodConfigNames.each { String key, Map<String, List<String>> value ->
        Map<String, List<ConfigAttribute>> configs = [:]
        populateMap configs, value
        methodConfigs[key] = configs // fails
        //methodConfigs.put(key,configs) // works
    }
}

If this could get resolved it would save a lot of plugins from having to be re-released with the workaround making less confusion about the error. In some cases this works and some it doesn't. Even though I have @CompileStatic set, the issue happens at runtime.