What should I use for Cubemap Dimensions? Power of 2 paradox/bug?

Hi, I've been using 512 x 3072 images to create skyboxes, setting the texture shape to cube and having Unity figure out which side is which based on this diagram from the documentation:

However, as of recently, I get this error:

Could not create asset from Assets/Visual/Textures/Skybox/Skybox_L2.png: Cube texture could not be created. Only Power of Two textures can be cubemaps. UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

I am using 2019.4.2f1. I used the same process in previous versions, such as 2017.3.0f3 also some other 2019 versions, although I'm not sure what the most recent one I used with this process was. But at some point it stopped working. I have old cubemaps in this project with the same dimensions that I set up earlier (in previous versions of Unity) that are totally usable for skybox materials, but now when I go to make new ones I cannot.

Is this a bug or am I actually using bad dimensions? The thing about the cubemap layouts is that they will always be aspect ratios of 1:6. 6:1, 3:4, or 4:3. which makes it not possible (i think?) for both length and width to be powers of 2. This is something I've always wondered about--how are we supposed to make power of two images that fit the suggested layouts?

Thanks!