We have the same issue with any onebox in a quote.
Adding an internal Discourse link gives us the “inline” onebox:
Oneboxes don't work inside quote blocks?
Twitter (or any other link) also doesn’t expand, we get “inline” too.
https://twitter.com/discourse/status/983464342799593478
It’s possible this broke as soon back when we added inline oneboxing, or a more recent regression, can’t say. Personally I’d call this a bug.