I recently updated my work computer to macOS 10.13 (High Sierra) and now find that all of my tabs within a single document are all named with the title of the document, not the focus item (search or tag) the tab is focusing on. I doubled checked at home where I am still running 10.12 (Sierra) and the tabs rename when you select a tag or a search in the sidebar as the basis for a new tab.
I’m running TaskPaper 3.7.6 at work (not sure what my home version is currently)