When leveling from lvl 3 to lvl 4, my lvl suddenly "jumps" to level 6 instead.

Also, the XP counter states "XP 0/1" (see attached screenshot).

Could look a little like an integer overflow?

If I kill another lvl 5 mob (40 XP), I go from lvl 6 to lvl 15! And the XP counter states: "XP 8/10".