"The man drinks up his beer."
Translation:Der Mann trinkt sein Bier aus.
You could think of it as an Easter egg, a surprise learning opportunity. Make a conscious decision whether to be annoyed or delighted.
FWIW I always keep dict.cc open in another tab, and when I got suspicious about "drink up" I thought about it and decided to look it up and there it was: austrinken.
"Up" in English is often used as a completive marker, even though most English speakers don't know that they're using it this way. Eg. "I fried the eggs" vs. "I fried up the eggs." If you fry up some eggs it means you've fried them completely and not just partially. Or "I shot the room" vs. "I shot up the room." Shooting up a room means you've shot everything that needed shooting, just shooting a room means you only shot it once.
Sorry for the grim example, but I hope that helps!
sein Bier is accusative here: it's the direct object of the verb austrinken.
seinen would have been appropriate for a masculine noun such as with seinen Orangensaft.
But Bier is neuter (das Bier), and neuter nouns ALWAYS ALWAYS look the same in the nominative and accusative cases, in all the European languages I know. So it's sein Bier in the accusative case just like in the nominative case.
In German, only the masculine singular looks different in the accusative case, since in German, feminine and plural nouns also look the same in the accusative case and the nominative case, not just neuter ones. (Whereas in Latin, Russian, Greek, etc. feminine and plural nouns are usually different nominative and accusative cases even if neuter ones are the same.)
Putting your money where your mouth is is a good idea.
Duolingo keeps track of learner retention rates. If they test a change and see people leaving, they will generally not make this change. If people stay but complain in forums which Duolingo staff do not read, I don’t think they care.
So stopping using Duolingo if you do not like it is a good idea, and probably the only way to send a signal to Duolingo.