I was recently playing Minecraft 1.16.5 with my friends, and I downloaded this Magic plugin for fun and it worked perfectly fine. I tried every feature of the plugin and found no issues. One day my friends joined the server and I showcased them the new magic plugin and they loved it. I told everyone to press Q to open the spell inventory, after they opened it the wand disappeared when they tried to close it. I don't know if its a bug, or we're doing something wrong. I hope someone can help me fix this. Thanks.

Does it happen every time?
Sounds like a bug if so, but not one anyone else is seeing. Could be due to some other plugin, do you have any plugins that would be doing something special when a player drops an item?

You can change the controls for your wand in the enchant table menu, try using right-click instead of drop if you can't figure out what is causing this.

Is it possibly AuthMe plugin is causing this? Or maybe its because I have a lot of plugins active. I do have a lot of plugins and I'll try to remove them to see if it works.

I doubt it's Authme, maybe ViaVersion if you have that but I also kind of doubt it.

More likely something that handles item drops, no idea what that might be.

If right-click works you can also change the defaults to that and maybe avoid the problem.

Is there any way for me to change the default button for everyone? For example G or a specific button?

No, the server doesn't read keyboard inputs like that from the game client.

You are limited to using Minecraft controls, left click (break), right click (interact), drop and swap keys.