More often than not, mirror tool refuses to work. Try mirroring landing gear pod (bulge on the left side of the fuselage) to replicate. It gives a message "Part symmetry is not available at this position". I'm not sure if this is a bug or a very annoying feature. Also, parts sometimes mirror into themselves using the part they are directly connected to as an axis of symmetry rather than command module/chip. I will try to replicate that if you want.
The symmetry mode works a bit differently than the SimplePlanes mirror tool, but it is working as intended so I am going to close this bug report. Symmetry is only available when connecting a part to the side of a fuselage. We have had some discussions internally about this because it does lead to confusion. At some point we will need to add a mirror feature similar to SimplePlanes.