I had this too. 
It just went into “pending” and sat there.
As a workaround, I cleared the browser cache, so that the “Delete entire sequence” button would reappear and I could quickly click on it before it could change into a “Deletion pending” label (again). In fact, the API effectively deletes images one‑by‑one because there is no true “delete sequence” request that passes a sequence key. In other words, the web app groups multiple image keys into one delete request.