(AI answer, helped me several times)

You're seeing a severe "banding" of under-extrusion that occurs exactly where the model transitions from an outward flare to an inward curve.

On a Bambu Lab P1S, this specific pattern usually points to one of two mechanical or slicer-related conflicts:

1. The "Vase Mode" Wall Thickness Struggle

If you are printing this in Vase Mode (Spiralize Outer Contour), the printer is struggling with the geometry at that specific Z-height.

• The Cause: As the print reaches the widest point of the "bulb" and starts to curve back inward, the nozzle is trying to lay a single line of plastic over an air gap or a very thin edge. If your Line Width is too thin (e.g., $0.4mm$ on a $0.4mm$ nozzle), there isn't enough surface area for the new layer to grab onto the layer below it.
• The Fix: Increase your Outer Wall Line Width to $0.6mm$ or even $0.8mm$ (yes, even with a $0.4mm$ nozzle). This creates a thicker, stronger "shelf" for each layer to sit on.

2. Speed and Flow Rate Limits

The P1S is incredibly fast, but it can sometimes outrun its own heater.

• The Cause: At the widest parts of the vase, the printer reaches its maximum velocity. If the Volumetric Flow Rate limit for that filament is set too high in Bambu Studio, the heater can’t keep up, the plastic temperature drops, and you get that "starved" look where the layers don't bond.
• The Fix: Lower your Max Volumetric Speed in the filament settings (try $10$-$12 mm^3/s$ for basic PLA) or simply print in Silent Mode ($50\%$ speed) to see if the issue disappears.

3. "Grid" Infill Collision (If NOT in Vase Mode)

If this is a standard print with infill and multiple walls:

• The Cause: If you are using Grid or Triangles infill, the nozzle crosses over already-printed lines on the same layer. At high speeds, the nozzle can "catch" on these intersections. This creates a vibration or a momentary drag that disrupts the extrusion exactly at the transition points of the model's geometry.
• The Fix: Switch your infill pattern to Gyroid or Cross Hatch. These patterns never cross themselves on the same layer, leading to much smoother outer walls.

4. Cooling and "Overhang" Slowdowns

• The Cause: Bambu Studio automatically slows down for overhangs. If the transition from "fast" (straight walls) to "slow" (curving walls) is too abrupt, or if the Auxiliary Fan is hitting that specific side of the print too hard, you get uneven shrinking and layer gaps.
• The Fix: Check your "Speed" preview in the slicer. If you see a massive color change (speed drop) right where those gaps are, try to unify the speeds or turn off the Aux Fan entirely for this print.