I'm having this same issue.
I installed a bunch of extensions without cron without issue. Then I read a bit about it it and enabled it. Now I have two extensions that have been pending for hours. One install and one uninstall. If I look at the history, I see this:
As you can see, matteocontrini/flarum-imgur-upload says it finished, but it still shows up with the spinner in pending.
The other extension doesn't even show up in pending. But if I search for "upload" it has the spinner in the results.
Here is the status on FoF Upload install task:
This has been going on for hours. What can I do?