The player's previous gamemode is not saved correctly on the server.

Expected behaviour (behaviour in versions prior to 1.19.3):

• /gamemode creative
• /gamemode spectator
• Press f3 + f4 (switches back to creative)
• /kill or rejoin the world/server
• Press f3 + f4 (switches back to spectator)

Observed behaviour (1.19.3+):

• /gamemode creative
• /gamemode spectator
• Press f3 + f4 (switches back to creative)
• /kill or rejoin the world/server
• Press f3 + f4 (switches to survival)

Code analysis (yarn mappings):
ServerPlayerInteractionManager#changeGameMode (1.19.3):

ServerPlayerInteractionManager#changeGameMode (prior to 1.19.3):

The previous game mode being set in ServerPlayerInteractionManager#setGameMode is no longer being set to the existing gamemode but instead the previous meaning that this never changes - it is always null, unless the player had set their previous gamemode in a version prior to 1.19.3.