I am not using Projects at all in Geany, so I was not aware that this might have any effect here. Now just out of curiosity I have clicked in the [Project] Menu the item "Apply default indentation" and ... Tabs now are inserted in the loaded Julia script file like in all other files. OK ... I don't need to understand this, do I? The issue seems to be solved by this click, so I am happy also without understanding why it occurred in first place and why now does it work as expected now after such click ...