Get a cell phone or tablet plug in bore scope. Look for gouges and ridge in the cylinder walls. It's not uncommon to start with intentions of pulling the heads to have valve work done or install new heads to find the cylinders need re done.
Yep. This happened to me. Looked in the bores with a camera. Saw some coolant in one. Turns out the 8553 gaskets weren’t sealing well (compression test was good) and coolant was seeping into the bores…pitted.