As the title states this seems to be a regression of the file permission issues. Unsure what's going as the Rakefile hasn't changed, but in a docker image with the gem installed it is unable to be used due to the permissions:
/usr/lib/ruby/gems/2.7.0/gems/fluent-plugin-detect-exceptions-0.0.14/lib/fluent/plugin $ ls -ltrh
total 24K
-rw-r----- 1 root root 4.5K Oct 5 09:04 out_detect_exceptions.rb
-rw-r----- 1 root root 12.6K Oct 5 09:04 exception_detector.rb