Failed compression in cylinder five

The best suspect would be a hole in the piston. You are right about this not being a ring issue. Even with no piston rings at all, you would have a lot of compression.

The best approach is to use an inspection camera and go through the spark plug hole to look at the piston. Next would be to perform a cylinder leakage test. A compression test only shows there's no compression. A cylinder leakage test will tell you why. The piston is placed at top-dead-center on the compression stroke, then compressed air is forced in through the tester. The gauge will show the percent leakage, and you can look in four places for the cause of excessive leakage. If the exhaust valve is leaking, you will hear hissing at the tail pipe. For a leaking intake valve, the hissing will be at the throttle body. Worn piston rings or a hole in the piston will be heard at the "oil" cap or dip stick tube. If the cylinder head gasket is leaking, you will see a steady stream of tiny bubbles in the radiator.