The page reloaded or the upscale stopped
That is the browser running out of memory mid-job, most often on a phone. The page resumes on the lighter engine; a smaller image or a computer avoids it.
When a browser runs out of memory it reloads the tab, which looks like the tool restarting. On the next load the page resumes your job on the WebAssembly engine, which uses less memory, and remembers that choice for two weeks.
If it happens again: close other tabs, try a smaller image, or run the same image on a computer. Cloud HD does the work on a server, so device memory does not matter there.