I discovered that my Frysk! points, which were supposed to be no ?, but have reappeared today.

example（my Frysk! point）：1583（before）,1583?(today)

Is it because I haven't played Frysk! for too long? Or is it a bug?

Yes, it's based on the rating deviation (you're "?" if you are >110)
When you don't play, little by little it goes higher and you can become provisional again