If I were to take this bug (probable), there is one of two approaches I could
take to resolve it.

The long way...
Remove sequential enforcement in both the level editor and the game. This would
allow for levels to be grouped by function (e.g. Debug levels could be 90 - 95
and tutorial level(s) >95).

The short way...
Use the level editor to enforce sequential integrity prior to saving/closing. If
any gaps are found, the level #'s above are shifted downward to fill the gap.

Take away point: level number is arbitrary! I'm aware of the wiki documentation
not relying on level number for sequencing but I'm not so sure about game details.