https://developers.facebook.com/tools/debug/

[rubixvi]

This is quite important.

There's no robot.txt file so there's no information being passed through from the mainsite to the shortlink.

This is occuring in the docker hosted version of Kutt.

Response Code Reason | This response code could be due to a robots.txt block. Please allowlist facebookexternalhit on your sites robots.txt config to utilize Facebook scraping

[trgwii]

I can't use this tool without signing in to Facebook. What does it say?

[rubixvi]

I can't use this tool without signing in to Facebook. What does it say?

Response Code Reason | This response code could be due to a robots.txt block. Please allowlist facebookexternalhit on your sites robots.txt config to utilize Facebook scraping

This is the error, basically Kutt without robot.txt won't allow facebook's scrapers to get the schema of the linked site.