We are in the process of updating a project from Material UI version 3 to version 4. As part of this update, JSS has been updated to 10. Ever since this update, the jss-increase-specificity plugin has not been working.
It looks like it was actually due to a change in how Material UI provides the JSS. I had to replace JssProivder with StyleProvider from material-ui. The plugin is still working properly.
We are in the process of updating a project from Material UI version 3 to version 4. As part of this update, JSS has been updated to 10. Ever since this update, the jss-increase-specificity plugin has not been working.