$ man bash | grep -C1 'trailing newlines'
       Bash  performs  the  expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the
       command, with any trailing newlines deleted.  Embedded newlines are not deleted, but they may be removed during word splitting.  The command substi‐
       tution $(cat file) can be replaced by the equivalent but faster $(< file).