FIXED!! The thumb shifter is connected to a hammer/trigger that drives the ratcheting gears. The spring that pulls the hammer into the smaller set of gears (in this case, 1, 2 and 3) needs lubricant and pressure to work the hammer back into position.

SO, 1) Remove all cable tension. 2) Remove the shifter cover plate. 3) Apply a drop or two of lubricant to the spring assembly where the thumb shifter is attached to the hammer. 4) Use a really small flathead screwdriver to push the hammer into the gears it's missing while the lubricant works itself in.

This may take several attempts, but just keep running through the gears and use the screwdriver to force the hammer into the teeth it's missing and it will eventually right itself. This technique saved five Shimano EF-29 shifters (left and right) for me. Good luck!