That had+continuous form always confuses me... I have to think to ”you had worked” versus ”you have been working”, or so... But you know better how to translate it, if it shows an action started in the past, and finished in the past before another action started and finished in the past too, then it is more like "băuseți/băuserăți".
”Beați” shows an action started in the past, but not necessarily finished when another action started in the past, about whose end (or not) we don't know. For which the proper translation is the one given by Duo. You were drinking cold water when we arrived. You are still drinking, maybe. Or you stopped after we arrived.
In Romanian, the two tenses are called ”more than perfect” and respective ”imperfect”