Stepper Moving in Bigger Steps FORWARD than BACKWARD

So, weird topic name, let me explicate!
Running Marlin on RAMPs, this is a new setup so isn't a recent issue, more of a new issue...
Other than the other strange quirks I'm trying to get rid of, the steppers (NEMA17 200s/r 59Ncm bipolar at 1/1 micro steps) will move FURTHER in the positive axis direction than in the negative for every unit distance told to move. This is regardless of whether it moves clockwise or anti-clockwise. To reiterate:
If I tell it to move +10mm in X it'll move +10mm in X.
If I tell it to move -10mm in X it'll move -1mm in X. Perhaps not exactly 1mm but you get the point!

Does anyone have any ideas?
Thanks in advance for the help

Is the gear tight on the stepper motor shaft? There's a little grub screw that needs to be tightened against the flat side of the shaft.

Any clicking from the stepper motor (skipping of steps)?

What happens if you go +10X, -10X, +10X?

Does this only happen in X? What about Y or Z?

Does this happen all the time? Or only after some time? (i.e. is this related to the temperature of the motor or driver board?)

Hi, I suspect it's more of a programming thing.

Both grubs are tightened fully.
No skipping.
It goes forward +10, back roughly -1, forward +10
This happens in every axis. NOTE: running CoreXY so both "X" and "Y" motors run together, they also run in opposite directions for X, i.e. Clockwise and counterclockwise. Yet it still moves in the different "scales". This persists even when trying standard and not CoreXY.
This happens all the time. Haven't even been able to do my first test print on this new rig (my second printer) so not related to overheating or power consumption. This is just happening as I manually move it either from the computer, OctoPi or LCD.

It sounds like a programming bug to me too.

I suggest you take this to a CoreXY or software forum, since I don't think it's related to stepper motors per se, so you're more likely to get a useful answer there.

I've filed an issue ticket on the firmware's GitHub so at least that's done!