I was trying to change the gamemode of people that died at least once to spectator (but only for people who aren't in spectator), so I used this command:
/gamemode spectator @a[score_dc=!0,m=!3] (with dc being the deathcounter)
and I noticed that apparently the m part (which is supposed to detect your gamemode) doesn't work with negatives, like:
this works as intended: /gamemode spectator @a[m=3]
this ignores the m=!3: /gamemode spectator @a[m=!3]

I hope this gets you enough informormation, it's easy to work around it, but I think it would be a good thing and probably easier to fix this