I think Bunpro could make use of implementing a 1 day grace period because things happen, you know. If it’s just a little over 24 hours due to situations like above, but the user still remembers their intention to do Reviews/Lessons/Cram daily, I don’t see why a user need to feel like they are not doing the work.
Not sure if this is pushing it, but 2 days grace period like Duolingo would be great too. Maybe having a deficit in XP for missing 2 days? But perhaps we can make a vote to see which would be more popular or beneficial, whether 1 day or 2 days grace period.
Anything more than that, the user just needs to be reminded of the use of Vacation mode.