How exactly does "override motion vector" block work ?

How exactly does the "attempt to override motion vector" procedure block work ? In some cases it works, in others it doesn't. I made a dash ability with a keybind and the procedure block works without any problem, but when I want to use it in a procedure executed when an entity spawns, it doesn't...

Are there any conditions for it to work ? Is that a client-side/server-side problem ? (I'm using 2023.2 and I can't upgrade to latest version because there is no 1.19.2 in 2024 version of MCreator.)