What macOS version are you on and when did this change? Did it change after you installed a Safari extension, made a Safari settings change, updated the OS, or installed another app?
it does the same thing. when unpinned we see the name of the site but at soon as you pin it the tab goes dry and you can pin multiple tabs and its all the same. We have tried pinning and unpinning, sliding it open and closed, closing it completely and opening, going to the window tab and pinning...it just does the same thing...gray pinned tabs with no icon.
I want to suggest you try removing all website data, but want you to know that it will log you out of some sites (including Macrumors). So you'd want to make sure your Safari login user names and passwords were all correct in the Safari preferences before you do that. (Safari > Preferences > Passwords).
When you're ready to remove some or all website data, go to Preferences > Privacy > Cookies and website data > tap on the Manage Website Data button. As a test you could remove a couple of the sites you know you have pinned tabs for to see if that makes a difference. To do that, select the site and tap the Remove button. You might need to repin those sites as well.
You can remove all of the sites too by using the Remove All button. I do this once a month or so to keep things cleaned up.