Why is COPY in docker build not detecting updates

I found this in the Docker documentation:

For the ADD and COPY instructions, the contents of the file(s) in the image are examined and a checksum is calculated for each file. The last-modified and last-accessed times of the file(s) are not considered in these checksums. During the cache lookup, the checksum is compared against the checksum in the existing images. If anything has changed in the file(s), such as the contents and metadata, then the cache is invalidated.

So, as far as I understand, the cache should be invalidated. You can use the --no-cache command-line option to make sure. If you get the correct behavior with --no-cache and an incorrect behavior without it, you would have discovered a bug and should report it.