Energy drains to zero between the end of lesson and returning to the path map (from 13 -> 0)

Android-whatever-is-the-latest-version.

I was going to finish the weekend quest (Complete 16 lessons) tonight and with full energy I was gonna get it done. My way is to go a specific adventure requiring the least amount of energy and for me it's one called "To the Museum!" in Spanish course requiring 4 energies. I had full energy bar so I can do the adventure six times (done that many times), whereas for the quest I needed only 5. However, at the end of the third run, I had 13 energies left, but when I returned to the path map, the energy had drained to zero.

Anyone else experienced this lately? For me this is the second time this week it happens. The first time I thought I just had messed something up, but now I happened to watch more closely.

And yes, I finished the quest with the free energy and watching some ads, but still quite annoying.