To fix this, there are two options:
You can go to Edit Devices, select the Axe-Fx and press Set Default Buttons, then press 1. Set Default Buttons didn't set up the Tuner button correctly in earlier firmware releases, but this will fix it. The downside of this method is that it will overwrite any changes you've made to your buttons on page 1.
or you can go to Edit Buttons, select the Tuner button, Press Add Action. Then set type to System and function to Tuner. Press done.
Either one should fix the tuner button for you.