Wow, I believe I overlooked an essential point. The testuser I created did not have the ftp checkbox ticked. As such ISPC always did reset the shell back to /dev/null by means of the 30 minute cron scripts. Not having this tick for a user means KEEP OUT
completely from this system. Which is exactly what it needs to do.
When setting the ftp tick box for this user, ISPC did set the shell to /bin/false as it should.
My humble apologies to the developers, as this function is doing what it supposed to do. Please ignore my suggestion to change the code. I'll stay low for a while .....