I also agree that situations like the one the OP pointed out are frustrating, but isn’t it to ensure that the user knows the material? The system doesn’t know anything about the individual circumstances each user brings with them nor can it read the user’s mind when they intended to type something else. Instead it can only reinforce responses that are correct.
So it would be understandable if you were asked the reading of a kanji review, and you typed an acceptable reading but it wasn’t the one being looked for, but then the review was marked wrong anyway. This doesn’t benefit the user whatsoever because it reinforces something that should be correct is unacceptable. However, with the case of vocabulary, there’s less flexibility; “ぎゅう” isn’t the way to say “cow” in Japanese. So to ask for a reprieve to provide a correct response doesn’t help the user learn that there’s a difference.
In general, I would rather get something wrong here and get corrected here, than to get it wrong when using Japanese and be misunderstood, laughed at, etc. by people in real life.
Additionally, this becomes less of an issue later on because there aren’t as many single kanji reviews/single kanji vocabulary reviews as one goes on.
One thing to take into consideration is the learning philosophy behind the design of the platform. It’s apparent that you have your own beliefs (philosophy) behind learning just like everyone else here. The decisions made with regard to handling cases such as giving second chances for kanji reviews but not for vocabulary reviews is informed by research the designers feel will net the best results in the long term. If a user could point to data to the contrary, I think it would effect how things are presented on the platform. But no matter how one frames it, there’s no data out there that says reinforcing undesirable behavior (i.e., in this case an incorrect reading of a vocabulary word) will net in a positive outcome in the long term. In other words, I don’t think the designers of this platform are going to weigh the opinions (i.e., subjective feelings)) of users to the same degree as what studies and data show to be effective with their approach. People losing their motivation and quitting is such a common occurrence that it’s almost impossible to prevent it. Not to say learner motivation isn’t important, but attrition due to a decline in motivation is really difficult to curb especially when learners have a high probability to quit before attaining fluency. However, the important part is the approach is effective for the learners who can get past these minor setbacks.