Oh I understand the confusion now!
It seems like you're closing a tab by swiping back, but actually that tab's navigation history had the start page as its initial page, and you navigated back to it, so it looks like a new tab.
Why it happens occasionally is a bit long to explain, but one scenario is when the app has an empty tab open, and a web page is open from another app, it's open in this empty tab instead of a new one.
It's been on my list for a while to remove that behavior 😅