This might be a duplicate of MC-172870 but I'm not sure, since I used movement manipulation via:

execute positioned as 00000000-4996-02d2-0000-0000499602d2 if block ~ ~ ~ minecraft:rail[shape=north_east] run data merge entity 00000000-4996-02d2-0000-0000499602d2 {Motion:[0.01,0.0,0.0]}

to move a minecart forewards when it's ontop of a certain rail.

I tried this command in a repeating commandblock in the always active mode, and it didn't detect the north_east rail. All others worked. But when typing it in manually/activating the commandblock with redstone mode, it worked.

This changed when my brother connected. Then even the always active mode worked.

The commandblock didn't generate an output when the minecart was ontop of the rail, which I think was due to the commandblock not detecting the rail. (In the always active mode before my brother joined). I wans't able to recreate it afterwards, so far.