Pro-Q 3 VST3 solo mode does not work on Harrison MixBus32c versions 5, 6, and latest v7 released on March 23, 2021. The VST2 works as intended.
When you are using the VST3 version of the plugin and click the Solo button a EQ band, the GUI updates but the sound is not affected. All other functions seems to work OK.
This has been reported in another post for Ardour. But since Ardour and MixBus share the same code they may be related.