This isn’t a permanent fix, but it does solve the problem on a temporary basis.
My first test was to restart DWM, which didn’t seem to do anything.
Next, I opened up Task Manager and hit End Process on the explorer.exe process.
Your taskbar previews should be back to normal, although you are probably slightly irritated that this was required.
After a few more times of encountering this, I’m going to do it myself.
