Random INVISIBLE character inserted mid-command in command block. Character can't be seen from within Minecraft so command appears correctly. CommandBlock reports successful operation but doesn't actually carry out instruction.

Command : /scoreboard players operation @e[type=ArmorStand,x=31,y=52,z=477,r=1] XXsm555XXTest4 = @p[r=3] XXsm555XXTest2

(notice character after 'y' in first argument - only visible when copied & pasted into an external text editor)

Isn't a consistant condition, i.e. haven't been able to replicate. Appears to be similar to the random/occasional insertion of the space at the start of the chat input that only seems to affect macs.

Once identified, delete and re-type section of code cures problem.

Note : Have seen before when things are c&p'd from text editor into game but this command was typed fresh in game, NOT copied & pasted.