I have the same problem.
All posts have featured images as thumbnails and all worked fine before the last upgrade (vers. 2.8).
Thanks for the attention.
Thread Starter
Asbam
(@asbam)
The issue seems to be a conflict with WordPress SEO by Yoast.
When just one of the two plugins is activated, the string in the tab is exactly as it should be.
I’ve tried to unselect the option “Force title…” in WordPress SEO and to restore default settings, but it doesn’t solve anything.