historically starlink’s satellites don’t always burn up in the atmosphere as they should

Any examples of that? My understanding is that they are in the same low orbit (400km) as the iss, and anything that low has to actively use some kind of thruster to maintain orbit.

The danger of Kessler syndrome really starts around 600km (orbit of Hubble) where deris can take decades to be cleared by atmospheric drag, and especially when 800-900km where it could take centuries.