The bug
In the player's NBT data, the previousPlayerGameType tag is used to store a gamemode ID. However, its name refers to the gamemode as the "game type", which is inconsistent with other mentions of gamemodes.
To reproduce
/data get entity @s previousPlayerGameTypeComments 3
Thanks for the insight 🙂
For additional context, I based this report off of reports such as MC-249370, which also describe inconsistent/old terminology being used in places.
There is also the GameType tag, which also refers to a gamemode. It has been named that way since Notch's days, and they have no reason to change it.