Those are very interesting results; however, I don’t think they give us enough data to draw a conclusion. When I mentioned cutting out a section, I meant to include part of the model that was smooth as well. Sorry if I wasn’t clear. Here’s what I meant:
Expanding the section to include both the clear and distorted areas helps determine if this issue is mechanical. Look for size changes in the distortion, did it get bigger or smaller? If there’s no change in size and the areas with good and poor flow are the same in both dimensions and location, that’s more conclusive. If the extrusion problems recur in the exact same location, this likely rules out the nozzle running out of molten filament due to travel distance and points us back to mechanical issues.
Another point—the circular test here doesn’t show the full travel pattern. Sticking to a symmetrical model print in circular pattern, like before, may yield better insights. This missing notch makes the test ambiguous and loses its value as a test.
Two Possible Alternate Theories
Theory 1: Bed Temperature Fluctuation
This is a long shot, but it’s worth ruling out. If you have an IR thermometer, check the build plate’s temperature consistency at the edges with and without the bed attached. ±5°C of variation is acceptable; greater deviations might suggest plate contact issues, affecting print quality.
If you don’t have an IR thermometer, they’re helpful for diagnostics and available for under $10 on Amazon or at auto parts stores.
https://www.amazon.com/s?k=ir+thermometer&s=price-asc-rank
Theory 2: Bed Warpage
There were early reports–most were debunked–some earlier beds had bowing issues. But Bambu did offer some folks bed replacements. Contact support and see what they think. The bed calibration was supposed to correct this, but it’s worth verifying that the bed itself didn’t warp. Try laying a steel ruler across the bed and look for any light peeking through. A small gap is normal, but a middle bulge causing the ends to drop by 1mm or more is not.
BTW: I know you said you ran the original initial bed calibration but have you repeated it?
Retro Test Using Painter’s Tape
Before heated beds, painters’ tape was commonly used for adhesion. Applying tape over the problematic area might offer additional clues. If you see improved filament adhesion with the tape, this leans more towards a mechanical cause.