Do all of the other switches work?

maybe switch 22 needs adjusting, or bad cap, or short on that line

How are you testing the switches above #22 in switch test if that one's always closed?

Go into switch test, disconnect coin door and see if 22 goes away.

If so you have short on a coin door switch
If not you have to isolate everything on switch 22 and trace it back to see where there's a sneak short (screws touching areas, frayed wiring, etc.)

Double check diode is connected correctly? Cut one leg of the cap see if that makes a difference. Leave disconnected until you resolve the issue.

As slochar questioned, how did you test the other switches? Only the lowest closed switch number will show up in switch test. Clear that switch before you can see what other switches are closed.

maybe try removing a wire from the switch and using your meter in diode test, test the switch?

red lead on + of diode, non banded end, (solder tag on the side, assuming this without seeing a pic) black lead on longest switch blade, (assuming this without seeing pic, one closest to playfield when in place) one that is not connected to the diode, but if a capacitor is installed, then that one (not the solder tag where the negative of the diode and capacitor join), when closed you should see a voltage drop of approximately 0.8 VDC

just to eliminate the switch from one of the possible causes.