Strange periodic artifact on inner and outer surfaces of a box  

Strange periodic artifact on inner and outer surfaces of a box

Hi. For this print I'm using my Prusa MK4 with the 0.2mm "QUALITY" setting, on the Generic PLA filament profile. The material is Bambu "PLA Metal Iridium Gold Metallic". I am using the regular print profile, not the Input Shaper, and the 0.4mm stock nozzle. Firmware 5.1.0, Prusa Slicer 2.7.1 for Apple Silicon.

Yesterday, I printed a simple rectangular box on this printer with these settings and filament, and it came out looking great. No issues. Today, I'm printing another box that is identical in design except its x and y dimensions are smaller and z is taller. All other printer settings are identical and it's the same spool of filament. The box has a slight taper (I think about 1°), wider at the top. It is also wider side-to-side (X axis) than it is front-to-back (Y axis).

As you can see, there are some strange little gaps in the outer layer of filament near the front left corner (as seen standing in front of the printer). There are similar little gaps in the inner layer near the front right corner. Looking at these with as much magnification as I can muster, it does look like a little hiccup in extrusion. I've attached as zoomed-in a photo as I could manage.

According to the slicer, for this print the outer layer starts printing at the corner indicated by the tail of the red arrow in the first photo (back left if you're standing in front of the printer), and the layer then proceeds counter-clockwise, in the direction the arrow is pointing. The inner layer starts extrusion in the front left corner and also proceeds counter-clockwise, as shown by the arrow in the second photo.

The upshot of all of this is that these little gaps or hiccups are appearing at very roughly the same linear distance traveled after extrusion starts. About 3.5" of travel near the bottom of the box on both inner and outer surfaces, decreasing to about 2.5" of travel near the top of the box. The taper of the box is nowhere near enough to account for this difference.

The entire rest of the print looks basically great. First layer is excellent. The other faces of the box (outer and inner) look smooth and consistent. Top surface (the "bottom" of the inside of the box) is smooth and consistent.

According to the slicer, the printer is printing the box walls with four layers -- two inner layers, plus the outside and inside of the box. It starts in the back left corner on the outermost inner layer (if that makes sense), prints that one, and then immediately steps out and prints the outside of the box. It then moves to the front left corner and does the other inner layer, then steps in and prints the inside surface. So in both cases, the surfaces that have the glitches start extrusion basically right next to where the inner layer ended, so the printer is not having to move far enough to even trigger retraction according to the printer's default retraction settings.

I have attempted this print twice with similar results.

I'm at a loss. What would cause this? Many thanks in advance.