As you know you only get a certain number of training sessions per level, and sometimes I might be doing something like going through a long dungeon and get multiple levels in the process, which means I lose one level's worth of training sessions. Additionally, a level up is a full heal, so I can lose the ability to use it strategically. The game even suggests "storing" a level up for the full heal, so it seems to be intended this way, however once a level up has occurred you have no way of tracking your experience anymore. Some people may see this as a trade-off for storing level ups, I see it as annoying. At least let me keep the added training sessions instead of just resetting them.

Note: While this says "Cancel Level up", a better name would be "Delay Level Up".