Why did the option (quick reply) buttons disappear?

Reply buttons can disappear when the shopper doesn't click on one of them, and then a new message from your page comes in. 

The new message probably came in because they commented again before they had a chance to click on the reply button for the variant option

And as you can see, the quick reply buttons disappeared.

It's not a big deal. But you can have your shoppers include the variant option in the comment itself. (ex "101 large"). Or just remind them to answer the questions from the sales bot before commenting again