Process group id of first command in command substitution (bash4 vs bash3)

[Roman Rakus]

I'm not sure if it's a bug or not, but there is change between old bash 
3.2 and bash 4.2.
When you run a script:
set -m
$(sleep 1; sleep 2)

in bash 4.2 the first sleep has same group id as parent shell. However 
in bash 3.2 it has different group id.

Is it bug or not? I'm not able to find documentation for this change. 
And seems that POSIX says nothing about it.

RR