Unable to run flatpak independent of the script that started it

I've always successfuly used an ampersand to start an application in the background from within a script. Such background applications seem to be detached from the script that started it, meaning that when I terminate the script, the applications keep running. I now have a script that first starts a number of applications, some of them flatpaks, and then starts an endless while loop to monitor my ip address to check that the vpn is still working.

#!/bin/bash
start application 1 &
start application 2 &
start flatpak application 1 &
start flatpak application 2 &

while true
do
   check ip address
   sleep 180
done

When I terminate the script with ctrl-C, the non-flatpak applications keep running (as expected), but the flatpaks are terminated. This, however, only happens if the script has this while loop: if I remove the while loop, then all applications keep running when I terminate the script, including the flatpaks. So far, I've tried: - calling the applications with the ampersand:

/usr/bin/flatpak run --command=de.manuel_kehl.go-for-it de.manuel_kehl.go-for-it &

- calling the applications with nohup:

nohup /usr/bin/flatpak run --command=de.manuel_kehl.go-for-it de.manuel_kehl.go-for-it &

- calling disown after starting the application

/usr/bin/flatpak run --command=de.manuel_kehl.go-for-it de.manuel_kehl.go-for-it &
disown

None of these approaches help. Can someone explain why this happens? And is there a solution? I would like to keep all applications, including the flatpaks, running when I terminate the script. tnx (using bash on Linux Mint 19.3 MATE)