If you right click on the tab, select 'inspect element' you'll get the dev tools open. From there...
you should see a <span> is selected - select its parent <a> tag
Select the computed styles tab
Filter the list for decoration
Hover over the text-decoration-line entry. It should have a value of none. If it doesn't, then click on the arrow next to its value to take you to where that css is declared.
But if something is overriding it, this should give a bit more of a clue where. It should have a link in the top right corner to take you to where it was added to the page. The hard part is due to the dynamic way we load node configs, it may not be instantly apparent where its coming from - but it may help you dig around.