this seems to be a problem in almost all powder simulation games considering mine had this same issue
so... what way is correct then?
Just dont fix it. It would break to many save. +0
The fix would be to make SWCH activate instantly, like TNT does. But right now, this happens because of particle order (which is a complicated subject), particles are only updated one at a time in a specific order, and down/right matches the order particles are evaluated in, which is why it is instant.
This probably has to do with evaluation order. Won't likely be fixed for quite some time.