Yes, dot behaviour is well documented.

But splitext() is a function to split name and extension, not take care about hidden files.

Hidden files starting with dot is Unix like system feature.
https://en.wikipedia.org/wiki/Hidden_file_and_hidden_directory

Hidden files could have also extension.
Try for example create/rename .somename.jpg file in Ubuntu.
It is normal image with .jpg extension. You could open it etc.

You can't use standard python splitext() function to detect extension.

I know that change this standard python function is probably imposible due to backward compatibility. Bud extend it with new parameter could be possible. The change in 2007 was unfortunate/incomplete.