You're right! It shouldn't be stressed as the robot has it. Please report it. :)
Though I fear they won't be able to do a lot about it!
I don't think the robot can be adjusted - you can disable listening exercises for a given sentence if the robot is particularly bad at them but you can't help it improve.
Can't they tune the stops and stresses? It seems to be strange, TTS systems usually have control codes or other means to do it...
Would it be written differently if one were to say "on Monday I drink beer"?