Player data, when stored to disk in either the playerdata folder or level.dat, stores various player abilities in the abilities tag. When editing these files with external tools, I would expect these values to apply when loading into the game. However, the majority are ignored and reset to their respective gamemode defaults.

Ability	Loaded from Disk
flying
flySpeed
instabuild
invulnerable
mayBuild
mayfly
walkSpeed

Provided is a world with an edited level.dat such that mayfly is set to 1, and the player's gamemode is survival.

Expected functionality:

The player is able to use creative flight in survival mode.

Actual functionality:

The player is not able to use creative flight in survival mode.