For some reason, when I try to store a player's look vector into a motion vector of another entity, I rarely get this error message that seems to only occur when any coordinate of the vector is an integer number.

Can you try using the floating point version 0.0d instead of the integer 0 when storing to NBT?
(It could be that Scarpet is casting the zeroes here to an integer data type instead of leaving them as Doubles)

I don't have physical control over the 0 exactly, so unfortunately I cannot do that. This is the line of code that's wrong, could it be that the "wspeed" value being an integer is causing this issue?

wprojectile_tag:'Motion' = (e~'look') * wspeed;

Do you get the same error if you replace the above line with the following (which assumes wspeed is 1)?

wprojectile_tag:'Motion' = (e~'look')

If you don't get the error, then it is likely caused by wspeed being an integer.

The issue still seems to occur without wspeed.

As a workaround, can you try filtering out those zeroes and replacing them with a really small floating point number (like 0.000000001)?

I already thought about doing that, but this is a bug nonetheless lol

Seems to actually occur when any of the three values are integer values, not just zero.

its all intended - and its a restriction of the NBT tags to store only one type in a list. In newer parts of MC points are stored as maps like {X:...,Y:...,Z:...} so its not a problem.
To do it properly, pass a string as value: