Merge subtool issue

Zbrush newbie here.

I’m having an issue successfully merging subtools. Merge down does not create a unified subtool the way it should. I noticed this issue after upgrading from 2020 to 2021.

Ex: If I perform mirror, only one part responds, not both. I’ve included example screens of how the sub tools look ‘merged’ and how they look after a mirror and weld.

Any help on this would be great. I’ve searched everywhere and I’m not sure what I did to screw this up.