Duo's speech recognition of numbers doesn't have the best reputation. But there's nothing the German course maintainers can do about it so there's not much point in mentioning it in the comments.
Better to contact the Duolingo developers directly, but I'm not sure whether they even developed it or just bought something from another company over which they have no control, either.
> Better to contact the Duolingo developers directly
You mean someone in the chain? Reaching out to some programmer directly might be not really effective.
Better to use "report a problem", and maybe some whining on the forum. I suppose developers have a list of tasks on their hands, and even 0.01% of users can overwhelm devs with feedback.
Thus they have a structured "report a problem" pipeline and, maybe, some sort of "community ambassador"s that monitor forums to know what's up.