on Python 3.6, the %X gives 14:09:44. Is this a intended or a bug?

Imho that does sound like a bug. %X should give a dot separated time, like 22.47.30

You can live test according to Linux man-pages project, release 4.14 at emil.fi/p/strftime (sorry about the css, it seems it's broken a little bit, you can use the "use desktop site" selection to have it behave better).