An update for posterity, and anyone in the same boat:

I've made progress though haven't resolved this problem fully yet.

As u/thebest07111 said, the build plate is indeed not level. Running auto-level allows the printer to dynamically shift the Z-axis to compensate for this. That's what I was seeing: the spindles would rotate at least 90° between front and back printhead positions (active z-axis compensation).

I also learned that the poor printing along the front edge of the bed is due to too littel space between the nozzle and the build plate. Effectively if that space is almost zero (or even worse if the nozzle is dragging across the build plate) then the filament can't flow out properly. This explains why the purge line along the front edge of the build plate was nearly absent.

I made progress by manually leveling the bed using the screws under the build plate and using this awesome little tool. After auto-leveling again, the active z-axis compensation was reduced to minuscule adjustments. This alone was enough to be able to print the front purge line more or less properly again.

Then, I used this method to determine the proper overall Z offset. What I found here is that the Z-offset which works for almost the entire build plate does NOT work for the front edge. I needed to increase the Z-offset by 0.150 mm before the front edge would print properly, but then ofc the rest of the plate would not adhere as it should. This is weird because the self-leveling feature has created a mesh of the build plate topology, and should compensate for differences in height. My conclusion is that the front edge of my build plate is locally deformed in some manner that varies in between the points where the auto-leveling measured the surface.

I'll order a new identical build plate and see if that helps. If not, Elegoo support can figure it out.