When you have a piston and command block as in setup.png and power the command block with a red stone signal it only triggers before the piston if you use a button and a direct red stone line. It does not trigger first if you use a repeater in the line. Does not seem to be directional.

Both command blocks have this command

execute if block ~-1 ~1 ~ smooth_stone run particle minecraft:happy_villager ~ ~1 ~ 0.05 0.05 0.05 0 10