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.
To avoid this, you can have your shoppers include the variant option in the comment itself. (ex "101 large").
This article goes in more detail about including options in the comment.
Or you can just remind them to answer the questions from the sales bot before commenting again.