Summary: If a Blueprint is created while not placed at the origin, the Location values default to non-0 and any future instance of that blueprint will ‘reset’ to those values. There is no way that we can tell to adjust those default values after creation and the fact that Reset to Default doesn’t set them back to 0 is unintuitive.
Drag an empty actor on to the level, at an arbitrary location. Optionally, set a rotation. Create a Blueprint out of this instance by using the Blueprint/Add Script button.
Now, drag new instances of your new blueprint into the level, and reset all the transform values by using the Reset to Default yellow arrows.
Notice how Rotation is reset to (0,0,0), Scale to (1,1,1), but Location will reset to whatever value was set when the Blueprint was first created. Trying to adjust those values via the Instance Modifications submenu of the Edit Blueprint button also fails. The blueprint has a way to set the default Scale, but nothing else.
Expectation: Seeing as we cannot adjust the default Location in an Actor Blueprint, it should default to 0,0,0.