Prior to 2.4.0, you could click the Geometry Add (or subtract, etc.) while holding Alt to get a compound, as documented in the Help: https://affinity.help/designer2/en-US.lproj/pages/ObjectControl/compound.html
If you pressed Ctrl instead it had no effect on the operation, and you just got a Curves object as though you had not used a modifier key.
In 2.4.0, pressing either Alt or Ctrl results in a Compound.
Recipe:
Create new document
Use the Rectangle Tool to create two rectangles.
Select both.
Press the Ctrl key and the Geometry Add icon on the Toolbar or Layer > Geometry > Add in the Designer menu.
2.3.1 will give a Curves object; 2.4.0 will give a Compound.
(Also applies to Photo.)
First mentioned as a question here: