I see this is marked as done and in the release notes for 0.99.131, however I am still hitting this issue in Version 0.99.131.0.1-beta.
As described by others above, when I use the back/forward buttons on my mouse (Logitech G304) the browser will navigate back/forward in history but if I am hovering a link, it will also open that link in a new tab. However I have found that if the tab I am on has no history, the back/forward buttons do not open a new link.
Another note is that when I have the AutoScroll chrome extension version 4.10 installed, I can see the scroll anchor showing up if I hold one of the back/forward mouse buttons. I can confirm the issue is still present even when uninstalling the AutoScroll extension and restarting the browser.
Please let me know if there is any more testing I can do to help assist with tracking this one down.