There has been a know glitch where it's possible to exploit creative mode by doing /gms, then /gmc, and then relogging right away. which turns their creative inventory into survival, it usually takes up to 20 times for it to 100% work, but it's very easy to do.
Can you please try to fix this asap? it's ruining my server's economy.

This was fixed a little while ago in a development build.

A way to fix this issue in the current build is turn off the delay when switching gamemodes.