SUMMARY
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If a custom Environment profile is created with a custom PYTHONPATH and the environment gets used in the Launcher, the PYTHONPATH will not get set.
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STEPS TO REPRODUCE
1. Go to Settings -> Configure KDevelop
2. Click on 'Environment'
3. Click the + (plus) button to create a new profile.
4. Name the profile 'Python_Test'
5. Add a Variable to the environment called PYTHONPATH
6. Set the value of the variable to a real custom path, for example: /home/myname/custom/module
To specify more than one path, delimit the paths with a colon (:). For example /home/path1:/home/path2

7. Click OK to exit the configure window
8. Click on the 'Run' menu and click 'Configure Launches'
9. Add a new launch by clicking the '+Add' button.
10. Set the Environment option to the environment we made in step 4 (Python_Test)
11. Click OK to save the Launch Configuration
12. Click the 'Run' menu -> 'Current Launch Configuration' sub-menu, and choose the Launcher that we made in step 9.
12. Run a Python script using the Launch Configuration that was just made. The PYTHONPATH variable will not get set.

OBSERVED RESULT
PYTHONPATH variable is not set from the Environment that is configured.

EXPECTED RESULT
PYTHONPATH=/the/path/i_chose:/second/path

SOFTWARE/OS VERSIONS
Linux: Ubuntu MATE 20.04.4 LTS

ADDITIONAL INFORMATION
It only seems to affect the AppImage version of KDevelop (v5.6.1). I tried using the Ubuntu package distribution version of KDevelop (v5.5.0) and the Environment variable set PYTHONPATH just fine.