I encountered the same conflict. Our issue is that when we select multiple categories, the radio button does not appear for us to choose the primary category. We tried to use another plugin, Select Primary Category (version 1.0) by Deniz ?zsen to force the selection of a primary category, but neither the permalink nor the breadcrumb in the front-end changes to reflect the chosen primary category.
So we thought the issue lies with Rank Math SEO (since we use it for the breadcrumb as well) and we contacted the plugin author. They suggested that I install the Health Check & Troubleshooting plugin to troubleshoot. So I did and I went through the tedious process of re-enabling plugins one-by-one.
In the end, we determined that Interlinks Manager is the plugin causing the conflict. The issue isn’t just with the inability to choose the primary category. With Interlinks Manager plugin installed, the Rank Math SEO meta box cannot be expanded in the Edit Post view.
