I don't know if it's a bug, but I've seen it happen several times where players lose % of their characteristics during the acquisition process, and you get a roughly equivalent XP boost (I think) in the % to be redistributed.
I don't know if it's a bug or if it's deliberate, and if it's deliberate I don't really see the point.
I think it's happened to me at least 3 times already

Hello, this is obviously deliberate, and was introduced at the request of the community when the new collective training system was introduced.

I'm a bit lazy to read everything :)
But if I understand correctly, when a characteristic has not been trained for a certain period of time, it goes back to zero but the % acquired is not erased and is put back into the common pot of XP acquired.
Is that correct?

OK thanks for the answers

After 30 days without training in stats, the % are converted back into individual XP

If I'm not mistaken, that's 70% of the state percentage converted into xp

40 days, I think, from personal experience.