When I use my Multimice if I press the key briefly then the key latches, if I press and hold it releases when I let it go.
There is this reference in the manual that seems to confirm the operation I see.
3.2.6 The “FUNCTION KEYS” are used to activate locomotive functions (F1– F28, e.g. the horn on a locomotive with sound or a digital coupling) by pressing the corresponding function key for an extended time (at least 1 second). The function remains active for as long as you hold the Function Key. If you press the key only briefly, the desired function is activated normally. You select the desired status using the arrow keys. Factory setting: OFF ▪ To confirm, press the OK-key. You return to the start level “FUNCTION KEYS.