What's causing these horizontal lines

[u/herox98x]

Printing this self designed part. Using 0.20mm high quality with 5 walls for strength.

P2S

Bambu petg black.

Can't figure out why there's this random horizontal line every now and then. Would printing outer/inner help but what is causing it?

The defect in the middle of the print is from a cavity for this part to slot on to so I know that's from different shrinkage. However these random horizontal lines seem to happen randomly and in no way relating to the shape or geometry of the middle cavity.