Modifying a global variable works, but not when a pipe is used

Question

Changing a global variable in a bash script works perfectly. But when a pipe is used it stops working as expected.

This works:

pf_test1:

#! /bin/bash

echo "1) MyGlobal=${MyGlobal}"
source pf_test2
echo "2) MyGlobal=${MyGlobal}"
change_global
echo "3) MyGlobal=${MyGlobal}"
inc_global 2
echo "4) MyGlobal=${MyGlobal}"

pf_test2:

#! /bin/bash
MyGlobal=100

function change_global() {
  MyGlobal=200
}
function inc_global() {
  RC=$1
  MyGlobal=$((MyGlobal + RC))
}

Output is:

$ ./pf_test1
1) MyGlobal=
2) MyGlobal=100
3) MyGlobal=200
4) MyGlobal=202

But changing the pf_test1 by adding a pipe, causes the the global variable not to be changed any more:

updated pf_test1

#! /bin/bash

echo "1) MyGlobal=${MyGlobal}"
source pf_test2
echo "2) MyGlobal=${MyGlobal}"
change_global | tee pf_ja.log
echo "3) MyGlobal=${MyGlobal}"
inc_global 2 | tee pf_ja.log
echo "4) MyGlobal=${MyGlobal}"

Output showing the issue

./pf_test1
1) MyGlobal=
2) MyGlobal=100
3) MyGlobal=100
4) MyGlobal=100

I don't understand why this fails. Any ideas?

And any solutions?

PS This is on Ubuntu

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 22.04.3 LTS
Release:    22.04
Codename:   jammy

Update: I changed the pf_test1 as shown in the comments:

echo "1) MyGlobal=${MyGlobal}"
source pf_test2
echo "2) MyGlobal=${MyGlobal}"
change_global > >(tee pf_ja.log)
echo "3) MyGlobal=${MyGlobal}"
inc_global 2 > >(tee pf_ja.log)
echo "4) MyGlobal=${MyGlobal}"

but I also added echo statements in pf_test2:

MyGlobal=100

function change_global() {
  echo "in change_global"
  MyGlobal=200
}
function inc_global() {
  echo "in inc_global $1"
  RC=$1
  MyGlobal=$((MyGlobal + RC))
}

The output is:

$ ./pf_test1
1) MyGlobal=
2) MyGlobal=100
3) MyGlobal=200
4) MyGlobal=202
$ in change_global
in inc_global 2

The output from the echos are done after pf_test1 ends. And the content of pf_ja.log is just the last line:

in inc_global 2