My WK- 3700 Casio keyboard is malfunctioning.

One portion of the key scanning matrix has gone bad. First thing is to clean the contacts with 99% isoprophyl alcohol. These USUALLY consist of traces on a circuit board that conductive rubber "pills" under silicone rubber domes are pressed down. Each note usually has two contacts, one closes before the other. This creates "velocity" measurement for the key. IF ONE of the contacts for a key doesn't work, the velocity can detect that the note has actuated, but no release occurs.

This would be on the "STROBE" line for either the first or second contact for THE OCTAVE you have problem with. There are disconnect diodes that prevent sneak paths.

You could have one bad diode, a crack in the circuit board, or a bad connection to the keyswitch board.