If launched from the main menu, Sandbox mode stores all designs in an another repository (a non campaign one). Not to worry though, you can always copy your designs from one repository to another:

• From the main menu, click the Repositories button;
  
• Then you can select your campaign repository on the left/right side;
  
• Select the other repository from the list on the opposite side;
  
• Just drag and drop unit designs that you wish to copy from one repository to another.

Hope that helps!

Thank you for the prompt answer! It indeed helps, I just organized the lists a bit and I see I can also finally alter the order and leave the experimentals and designs for fun at the end after the more useful ones, that was also bugging me.
This would be welcome in the tips section.

Also cheers for having what it takes to make a game this unique, with such tremendous potential for quite a niche audience