Schema.org 'specialty' property failing with Q&A type: "The property specialty is not recognised by Google for an object of type Question." - schema.org

Question

I'm trying to add the specialty property, following the Schema.org docs using JSON-LD. According to the QAPage docs, specialty is indeed a valid value for this schema type.
The problem is when I validate with Google's Structured Data Testing Tool, it spits out this error:
(The property specialty is not recognised by Google for an object of type Question.)
Now, Schema.org is partly ran by Google, so I don't feel it's an error on their end. Pretty sure it may be JSON-LD syntax but I've been trying different things all morning. Could anybody shed any light on the issue?
{
"#context": "https://schema.org",
"#type": "QAPage",
"mainEntity": {
"#type": "Question",
"name": "How many ounces are there in a pound?",
"text": "I have taken up a new interest in baking and keep running across directions in ounces and pounds. I have to translate between them and was wondering how many ounces are in a pound?",
"specialty": "SEO", //ISSUE LINE
"answerCount": 3,
"upvoteCount": 26,
"dateCreated": "2016-07-23T21:11Z",
"author": {
"#type": "Person",
"name": "New Baking User"
},
"acceptedAnswer": {
"#type": "Answer",
"text": "1 pound (lb) is equal to 16 ounces (oz).",
"dateCreated": "2016-11-02T21:11Z",
"upvoteCount": 1337,
"url": "https://example.com/question1#acceptedAnswer",
"author": {
"#type": "Person",
"name": "SomeUser"
}
},
"suggestedAnswer": [
{
"#type": "Answer",
"text": "Are you looking for ounces or fluid ounces? If you are looking for fluid ounces there are 15.34 fluid ounces in a pound of water.",
"dateCreated": "2016-11-02T21:11Z",
"upvoteCount": 42,
"url": "https://example.com/question1#suggestedAnswer1",
"author": {
"#type": "Person",
"name": "AnotherUser"
}
}, {
"#type": "Answer",
"text": " I can't remember exactly, but I think 18 ounces in a lb. You might want to double check that.",
"dateCreated": "2016-11-06T21:11Z",
"upvoteCount": 0,
"url": "https://example.com/question1#suggestedAnswer2",
"author": {
"#type": "Person",
"name": "ConfusedUser"
}
}
]
}
}

Answer 1

The specialty property needs to be added to WebPage (or one of its sub-types, like QAPage).
You added it to Question (where it’s not listed).