SNM

This isn't a bug and isn't unique to level 80. When you die you lose 4% of that current level. If that causes you to delevel, your total xp loss will be 4% of the total xp required to level for the level you were when you died. The reason you haven't noticed this issue before is because the difference in xp between levels wasn't that as dramatic as it is at higher levels. For instance, if you died at level 79 and 1% you would lose roughly 890m xp (4% of the required xp to go 79-80). That would be enough xp loss to delevel and go down to roughly 90% at level 78. In short, do everything in your power not to die with less that 4% progress into your current level.