What sort of debris has been found in 737 Max fuel tanks?
As a generic answer, not unusual. Besides personal objects like bucking bars, clecos, rags and such left in tanks, the biggest contamination source, from my experience working on a production line many many years ago, is "swarf". Swarf is aluminum drill shavings and mountains of it are created during assembly.  You will also get wire cutoffs and other similar bits, shop (bucked) heads of rivets that were drilled and punched out because the rivet was dumped when being driven and had to be removed and reshot.  

All that stuff is supposed to be thoroughly vacuumed out before the sealers go in with their goo guns, but nothing is ever perfect.  Even if the wing was cleaned out before the sealing was done, if the wings get modded or repaired farther down the line, where more rivets get drilled out and redone, more swarf or other contam gets introduced and may get overlooked.  

The first time fuel flow test runs are done on the completed aircraft, that's when you find out how bad a job they are doing up the line as the fuel screens and filters are removed to check.

Some manufacturers will build special rotation fixtures to put the wing in (not really practical in a one piece swept wing, but easy to do with a straight one) after it comes out of the assembly jig to rotate, barbecue chicken style, while they listen for objects clattering about. It's good for catching bucking bars, clecos and such that get left inside.