Tabs remain in sidebar even after closed in browser, only on Linux

I have been happily using this extension for a few years, but a few months ago it started a behaviour that's rather annoying. I'd initially thought it was some new feature that I never managed to learn about. However,  when recently using Windows for the first time in close to a year, I'm thinking this may be a Linux-specific bug. Sidewise on Windows behaves like I expect, but on Linux (both Chrome and Chromium), tabs need to be closed in the sidebar to truly disappear, i.e. closing tabs in the browser leaves the showing in Sidewise.

Is this a bug, or am I missing something?