is_author() for the post

Have you tried selecting the wp_reset_query option at the bottom of the main widget page? Give it a shot, it could be your theme, selecting this option will sometimes fix the issue.

I also read somewhere that the plugin does not seem to work properly with the new WP release, not sure if that issue has been resolved or not.
I personally cannot get it to work at all

is_author “checks if an Author archive page is being displayed”

https://codex.www.ads-software.com/Function_Reference/is_author

Do you want to show a widget to a logged in user who is the author of the displayed post?

There is a global $userdata, and $userdata->ID is the ID of any logged in user.

I don’t know what the simplest way of getting the post author ID is, but a long way round is to use the global $wp_query, something like…

$author_obj=get_userdata(($wp_query->post)->post_author);

not sure if you can do it as concisely as that you may, you may have to break that down into evaluate each -> first. Then when you have $author_obj and $userdata, you can use

return ($author_obj->ID)==($userdata->ID);