Last site visited appears as a Top Site on startup page

I have pinned six top sites to the startup page. I’d like these top sites to remain. But several times a day, after I’ve been browsing for a while, I bring up the startup page again and the first top site on the left has been replaced by an icon for the site I just visited. If I delete that top site icon, the previous top site (the one I wanted in that position) comes back.

I tried reproducing the problem by simply visiting a site and then going back to the startup page, but the problem didn’t occur. So I don’t know if there is a pattern to this problem.

I’d prefer that once a top site has been pinned, it stays on the startup page until it’s removed. Thanks.

Version 1.3.115 Chromium: 80.0.3987.87 (Official Build) (64-bit)

PS: It just happened again. This time the new site that appeared was Brave itself. That resulted from visiting the main website and then the community. So maybe the problem occurs if you visit a new site a certain number of times, even though you’ve pinned all six top sites you want and don’t want them replaced.