I’m experiencing issues with the quote function as well. It is not entirely consistent though and therefore not entirely duplicable. Different users report experiencing issues as well.
When I tried to quote using my iPhone (something I’ve certainly done before), it would now only let me quote a single word; after I selected and highlighted more text beyond a single word, the prompt to quote would not appear.
When quoting on my laptop, it seems rather inconsistent and I haven’t noticed a pattern. Although, if I double-click / highlight a single word, the quote function does seem to appear consistently. The issue seems to exist when highlighting / selecting text beyond a single word, but inconsistently so.
We had updated to the most recent version of Discourse (2.6.0.beta3) not long ago, so it would seem this issue may be a bug in the most recent update.