I’m seeing this too - Firefox, Linux, Sam’s Simple Theme.
Perhaps I should check out some more of the themes on offer… never experimented with them!
They’re easy to try and easy to revert. Note that they differ greatly between PC / tablet and phone. So if you use both devices regularly, you’ll want to test any theme on both devices before making a decision.
It just happened to me on my computer, using Brave browser, and SD Light theme. Refreshing the page fixed it.
The only time I’ve seen this behavior (SD Light) is when I had an internet connection issue, even a very brief one.
I should note - my workaround for when this happens is to use the scrollbar displayed on the right side of the screen to move down a few posts, then scroll back up to where I had just left off. A bit irritating, but workable.
This is what I do. And I use Vincent.