Recently, it appears that two directories for 8.4 and 8-stream are being
distributed in the Centos mirrors.

You can't see it if you view the mirrors, but if one downloads the
filelist.gz file and grep for '^\.\/\.' one will see the directories, with
content, .8.4.2105 and .8-stream.

Can this be explained? An error or a new distribution concept??

If the latter, then this may have an impact on those who have off-line repos which they manually rebuild.

Rgds