/defaultgamemode

Sets the default game mode for new players joining the server. Does not change existing players' game modes.

Syntax

/defaultgamemode <mode>

Name	Type	Required	Description
mode	gamemode	Required	The default game mode: survival, creative, adventure, or spectator.

/defaultgamemode adventure

Sets the default game mode to adventure for all new players.

/defaultgamemode survival

Sets the default game mode back to survival.

Expecting /defaultgamemode to change existing players' modes, it only affects newly joining players.
Confusing /defaultgamemode with /gamemode, use /gamemode to change a specific player's mode.

This changes the default-gamemode setting. Existing players retain their current game mode. To change all current players, use /gamemode.

Try these commands on Astroworld or host your own server.