The clean opt can not delete all objs(like *.o)

[chrlis]

I find a special case:
->cd bash-4.4
->mkdir build 
->cd build
->...
->make clean

When I make clean current building directory, it never remove the object 
'man2html.o' in the path /path/to/build/support.
Is this a problem ? 

The released bash source code version is: 
https://ftp.gnu.org/gnu/bash/bash-4.4.tar.gz