This appears to be broken in the betas. I'm using beta 6 (I didn't try it until the betas, so I don't know if it ever worked in Godot 4). Does it still work for you?
@export var m_vecSize:Vector3 = Vector3(1,1,1) : set = setSize, get = getSize
m_vecSize = a
Neither getSize() nor setSize() get called when changing the value in the editor. The editor shows that the engine recognizes the change, but my set and get methods don't get called then.