Does Imagick remain selectable in the options? What about in XF Attachment Options? Generally if it reports it's not installed... it's usually not wrong or, at least, it's not there when we're looking at it so it's indicative of some sort of external (to XF) issue.
I would guess you just need to go to somewhere that allows you to configure PHP extensions for the current PHP version. If that appears to be enabled, but it still isn't showing as enabled in phpinfo, then it is almost certainly a host question.