Gary Illyes from Google confirmed he is slowly working on improving the robots.txt tester tool, which is currently available on the old version of Google Search Console. He said on Twitter "we're currently in the process(*) of integrating the tester with the production version of the parser."
Gary said this because there was a complaint that there is a bug with either the tester tool or how Google reads a site's robots.txt file. Gary said it might be just the tool itself and when the tool is updated to use the production version of the parser, you will know for sure.
Here is Gary's tweet:
we'll look, but it's likely a tool error. we're currently in the process(*) of integrating the tester with the production version of the parser which should fix this, if indeed it is a tool error.
* - it's my procrastination project, so no holding breaths— Gary 鯨理／경리 Illyes (@methode) January 6, 2021
If you scroll up through the Twitter thread, you can learn more about the complaint.
In 2014, Google revamped its robots.txt tester tool but it has been updated numerous times before and after that time.
Forum discussion at Twitter.