As a temporary expedient I'm going to avoid using the tab key and stick with spaces.
Mmm, that shouldn't matter if, when loading, Lazarus makes them Tabs again. But if it does, then you didn't explain the problem correctly. I thought you saw spaces before saving and a tab after loading.
Could you do the following?
Under Tools > Options > Editor > Misc: Set the "
Show special characters".
Now you can see, if you use the tab, if there is really a tab set (it shows » character as tab) or the tab is converted to spaces (· characters). What do you see?
(It's important to know if it has a tab
before saving.)
If it has tabs, your "Tabs to spaces"-setting doesn't work.
If it does have spaces, save the file and look with a bin/hex-editor if the spaces are $20 of $09. Which are they?
Now load the file again and see if the spaces are converted to tabs.