Rather a long one, but:

A Command Block (such as powered via a redstone clock, or set to always active) will detect an output for test gamemode (so [m=]) only if the gamemode is activated in-game via another Command Block. If a player sets their gamemode via the command line (by pressing T) the block will not detect this – even if triggered directly by redtone after typing the commandline.

I’m sure this never used to be the case, as I’ve used this in the past (I’m sure I have)