Suspended child process exit if parent process exit

I have the following process tree

test1.sh
  \- test2.sh
    \- sleep 600  

Normally If I kill the test1.sh process, the child processes test2.sh and sleep 600 will continue running. But If I suspend the sleep 600 process through send signal (SIGSTOP or SIGTSTP), and then kill the test1.sh process, the child test2.sh and sleep 600 will exit. Why?

Here is my test program:

test1.sh

#!/bin/sh

./test2.sh

test2.sh

#!/bin/sh

sleep 600

Test steps:

1. run test1.sh

   $ ./test1.sh

2. open new console and suspend the child process.

   $ kill -19 < sleep pid > or kill -20 < sleep pid >

3. kill the parent process test1.sh

   $ kill < test1.sh pid >

You will find the after step3, the test2.sh and sleep 600 exited.

Bug if I only run step1 and step3, ignore step2, the test2.sh and sleep 600 process will not exit.

Can anyone explain this? Many thanks.