I have a (non-production) Discourse forum running 2.0.0 beta 4. We have imported posts which include quotes-within-quotes (i.e. a post quotes another post which itself contains a quote). This does not display properly as shown below. This bug appears to be the same as this bug from 2014.
(Both the post being directly quoted and the post quoted within it do exist on that post so it’s not an issue there)