The JTAG pins doesn't belong to any of the groups mentioned in the list.
This is just a limitation in the mt7688_pinmux package. The mt7688_pinmux package that is provided doesn't handle the pinmux for all the pins that support PIN sharing.
Following are the pins and the groups for JTAG pins.
If you want to change the GPIO mode of JTAG pins, you have to rewrite/extend the mt7688_pinmux package.