This is not a bug. We had adjusted how our leveling system works in Patch 2.1.58 released yesterday.
Level 1 - 25: 1,000 XP each
Level 25++: 20,000 XP each
You did not lose any experience points that you have gained from playing. It is just that the calculation for levels is different. Due to this adjustment, everyone's level have been recalculated to reflect the change accordingly.
😃