Dont know what the principal is, but that is how it’s always worked in FS series. You hold trim key down for 1 min and want to undo what you just did just a key press wont undo it, it has to be a key press equal to the time of the opposing key press.
There was an excellent tutorial on this back in the FS9 days (well it was a take off tutorial), but he went into this fact and I have always found it to work this way. If you watch your trim wheel while you pressing the keys you can see it in its effect.