eshanized up to you, you may have a try if you want to. Somehow, the presence of this issue is not due to any bug/error in the extension manager package itself which mean you might not encounter the same or maybe any error, as there is something wrong at my side here in which either it is in my flarum script or something is missing or restricting at the server-level which is causing the extension manager unable to operate as intended.
Anyways, seems you're able to conduct activities via CLI, i would suggest you to remain with it.