Why am I getting Z banding/shifted layer on a single layer? While I was calibrating my flow rate/extrusion multiplier, I noticed that one layer is shifted slightly. I wanted to re-print the cube in-case it was a fluke, but after two more prints, I saw that the same layer was being shifted in the same way. Also, that is the only layer that is shifted, every other layer is perfect/almost perfect.

I'm using Cura 4.3.0, on an Ender 3 which I purchased about a month ago. The extruder is upgraded to a Trianglelabs Bondtech BMG clone. The E, X, Y, and Z steps have all been calibrated. The bed is level (I'm using a Creality ultrabase glass bed). I have upgraded the stock PTFE tubing to Capricorn, upgraded the PTFE coupler fittings (they have plastic tab inserts to keep the tube in place). The X gantry is level. I've also re-lubed the z-rod with super lube, made sure the lead screw nut is loose and printed out a z-spacer for the z stepper motor. That's about it, I'm still using the stock board but my SKR mini is in transit right now. Finally, the filament I'm using is the filament that Creality bundled in with the printer when I purchased it off their official website so I don't know how quality their filament is.

Below is a picture of what I'm talking about, I've highlighted in the area with a sharpie. Sorry for the bad image quality my phone is on its last legs at the moment haha:


This is the guide I was using to calibrate the flow rate and also where I got the STL
I started this as a comment but it couldn't fit.  If it isn't a true answer and should be removed, I understand.

You suggest in your question that perhaps the defect could be caused by a problem in the Z-axis lead screw.  Indeed, that is possible.  If so, it would probably introduce a defect at that position across the entire print bed.  If you print a test object and the defect is on all faces at the same height, check your Z-axis.  With the power removed, try slowly moving the bed up and down through the region where the defect occurs, feeling for any hangup, hesitation, looseness, or any other non-uniformity.

If the defect is along a particular line, check the complementary axis.  If the glitch is a X-axis line, check the Y-axis.  If a Y-axis line, check the X-axis.  Slid the axis back a forth slowly, feeling for any non-uniformity in the motion.  It could be a scratch on the linear bearing, or a ding from dropping something.  Any discontinuity  can cause a linear Z-axis defect, up or down.

It could even be cause by the printer not being firmly sitting on the table at an angle, so that after a particular point the balance point changes and the printer rocks, introducing a glitch.

3D printers, and many other motion control systems, relay on flat, smooth, uniform bearing surfaces connected to smooth, continuous drive systems.

If you attach photos of all four surfaces, we might have a better idea of how to look for the problem.

It wouldn't hurt at this point to wipe and clean all sliding surfaces, and then re-lubricate them.