It’s probably still in the Relative transform mode while you will want Absolute Location & Rotation for this:
This may have more to do with the size of the world; float precision could be a culprit here and is a normal behaviour. You’d need to include more details here - what the worlds is made out of and what distances we’re dealing with. But this is probably unrelated to the spline itself, though.
It’s part of the engine:
But you should be using your own mesh and your own material for this. This one is not suitable. The little demonstration above nothing more than a proof of concept and should not be used as is.
Here’s a another, slightly different approach - this one takes into account the player standing still and creates points only if you’ve moved far enough from the previous location.
This starts on Begin Play. You could play with tangents manually but it may not be necessary, depends on the desired end results:
This is just the spline, now you need to hook it up to a spline mesh component. Do a quick YT search - this is something covered extensively. One way or another, this will need a lot more work to get a usable state, for sure.