Sorry to say, it sounds as if you have lost ring seal. TRy to squirt a dash of
oil in the cylinder than showed lo compression, spin the motor over 2-3 times and recheck--is your compression back? If so, how long does it last? If you are very lucky, than that cylinder is running too rich and merely lost ringseal (the
oil barrier that seals the rings to the cylinder). If you are like me, you lost a ring and will have to pull that piston and put on new rings. SORRY.
Usually a burnt valve will be a regular, hard miss but it's easier to check valves from the bottom half, after you remove the piston. If you dont have a borescope...good luck, and I hope it turns out simple and quick!