About half the time recently when printing with PLA, I see that holes or loops have a line of filament running approximately through the middle which seems to be one of the inner perimeters having detached and contracted, but is still attached on both sides. Is it a temperature thing, an extrusion thing? I can't find a pattern. Bed adhesion is great. Bambu H2C, mostly printing with Bambu Basic PLA.
Nothing ever pulls off from an outer perimeter, it’s only inner perimeters of empty loops or holes.
Isn't that obvious?
Try making a circle of a certain size on a flat, empty table, by dragging a string. The circle will become smaller, than the target size because of the part that is already laid down will be dragged inwards.
Now drag the string around a gluestick, it can't be smaller than the gluestick's circle because the gluestick is in the way.