forked before bash subshell

[s7v7nislands]

1. the script to reproduce
bash-4.4$ cat t.sh 
(cd /tmp && sleep 20) &

echo "end"


2. run it
bash-4.4$ bash t.sh 
end

3. script end, but there is a new forked script.
bash-4.4$ ps -ef | grep t.sh
  501 50268     1   0 10:09AM ttys015    0:00.00 bash t.sh
  501 50275 50181   0 10:10AM ttys015    0:00.00 grep t.sh



question:
when using sh -> dash, It will not fork before subshell, what's the different?

why bash bahavior like this? is that a bug?

if not, why should forked before execute subshell?

thanks!