"This banana is expensive."
Translation:هٰذِهِ الْموزة غالْية.
Do you mean your question is like follows: (1) a banana -- مَوْزَةٌ "mauza(tun)" and (2) the banana -- المَوْزَةُ "al-mauza(tu)"?
Generally speaking, in Standard Grammar:
a). If it is indefinite (without ال), the ending sounds are -an (in the accusative case), -in (in the genitive case), and -un (in the nominative case).
b). If it is definite (with ال), the ending sounds are -a (in the accusative case), -i (in the genitive case), and -u (in the nominative case).
For the sentence above, we pronounce "al-mauza(tu)" because the word is in the nominative case (as 2al-mubtada2). Is that your question?
Nb: However, Arabic Duolingo contributors try to omit almost all ending sounds and don't completely follow Standard Grammar rules. So, I guess it is a bug.