Using the following example:

dictionary TestDictionary {
  sequence<CompositeOperation> seq_member;
  (CompositeOperation or sequence<CompositeOperation>) union_member;
};

For seq_member, V8TestDictionary::ToImpl will call into NativeValueTraits<IDLSequence<IDLString>>::NativeValue for the conversion, then call IsValidEnum to check that the values are valid:

void V8TestDictionary::ToImpl(...) {
...
      Vector<String> seq_memberCppValue = NativeValueTraits<IDLSequence<IDLString>>::NativeValue(isolate, seq_memberValue, exceptionState);
      if (exceptionState.HadException())
        return;
      const char* validValues[] = {
          "replace",
          "add",
          "accumulate",
      };
      if (!IsValidEnum(seq_memberCppValue, validValues, WTF_ARRAY_LENGTH(validValues), "CompositeOperation", exceptionState))
        return;
      impl.setSeq_member(seq_memberCppValue);

...
}

For union_member, however, V8CompositeOperationOrCompositeOperationSequence::ToImpl does not do an enum value check in the case that it is a sequence:

void V8CompositeOperationOrCompositeOperationSequence::ToImpl(...) {
...
    if (HasCallableIteratorSymbol(isolate, v8Value, exceptionState)) {
      Vector<String> cppValue = NativeValueTraits<IDLSequence<IDLString>>::NativeValue(isolate, v8Value, exceptionState);
      if (exceptionState.HadException())
        return;
      impl.SetCompositeOperationSequence(cppValue);
      return;
    }
...
}

I think there needs to be a call to IsValidEnum before the impl.SetCompositeOperationSequence(cppValue);