What's causing this?

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.