Had similar issues to another user. Upon opening new tabs from existing tabs, they always went to weird places (to the far right usually) instead of opening next to the tab I was using. This made the addon quite unusable to me...

BUT, I found this can be changed in Manage addon -> Options. At "Tabs opened from Existing Tabs", change "Last Child of the parent tab (recommended)" to "Child of the parent tab, next to the recently opened child (simulates the browser's default behavior)".

So I will continue using this for now. However, it might be a good idea to edit the default so it's more similar to the browser's default, I feel like most people can't be bothered to dive into the options/settings.