BIS_fnc_MP does not appear to fire when there are more than 10 switchable units on the map and this is passed as an array of objects to the function. e.g.
["Hello World 1","hint",(playableUnits + switchableUnits)] call BIS_fnc_MP;
Will output no hint to the player in single-player testing. Having less than 10 switchable units present appears to process the command correctly.
This was not an issue before the 1.54 update. {F27463}