"I'm attending" button keeps showing even if a user is already attending

Create an event and click on "I'm attending" as logged in user, you'll be successfully added to the attendee list.

but the "I'm attending" button will still be there.

If it's an "unlimited" even, each next click will give you the same "success" message, while
the user will still be added as a single attendee. With "limited capacity events" it will either do as above or show that capacity had been reached. The button will stay there tho.

After the user clicks "I'm attending" the button should become inactive or change to "cancel" option.

That looks like a "glitch" or rather a bug in the plugin.