Why doesn’t the viewfinder focus match the photo when I reverse a lens for macro?

Reversing the lens, when doing close-up photography is a valid technique. Non-macro lenses are optimized to image objects that might be at infinity or as close as 1 meter distance or anywhere in-between. In other words, the typical subjects are not flat but have depth. Conversely, the macro lens is optimized to image flat subjects like stamps etc. When the non-macro is tasked to do macro work, we mount the lens forward of its normal positon using a spacer called a tube or bellows. This allows the non-macro to focus on subjects that are only a short distance from the front element of the lens. Often the resulting image is substandard because the non-macro is not optimized to image flat subjects.

Revising the lens points the rear of the lens at the subject. This end of the lens is optimized to handle a flat surface like film or digital sensor. Often higher acuity is realized. Additionally, often some additional magnification is achieved because the focal length measuring point within the lens barrel (rear nodal) is not at the center of the barrel. Reversing the lens shifts the rear nodal position and this adds a few millimeters to the forward reposition.

Now the diopter adjustment you are asking about is strictly an adjustment you make to correct for your eyesight and allows you to work the viewfinder without using your eyeglasses. I can think of no reason why reversing the lens should impede this. I think, if the diopter adjustment is proper for your eye, what you are seeing truly depicts the acuity of your cameras focus. In other words, if reversing the lens causes a degrading, what you are seeing is actually happening, you are not achieving tack sharp focus.

Addendum: Likely what is actually happening – you are composing and focusing with the lens aperture wide-open. This is as it should be because otherwise the image would likely be too dim. Additionally, with the aperture at maximum, the depth of field is at its shallowest. Likely the taking aperture is stopped down; this act increases sharpness. I think this is the real reason your viewfinder view is substandard compared to the actual image achieved.