Lack of information
I don't know if it's an update, a glitch, or an A/B test, but suddenly Duo is no longer telling me what skill(s) I've strengthened after I do review. If it's an update or A/B test, I'd greatly appreciate being given that back, please. It was very helpful, and now it's gone for no apparent reason.
EDIT: It's back! Thank you, Duo staff; I know it seems like a small thing, but it's so very helpful.
I know, I also posted something here instead of the Troubleshooting forum a while back for the same reason. But since it's more website than language related I think it belongs there, since the general Forum is more by users for users and there seems to be an admin going around replying to some of the posts in the Troubleshooting forum (and hopefully at least reading the rest), some of the mods also seem to know if something is an A/B test or not.