Not scientific but I put a bit of light absorbant material in the 28mm extension tube and took some shots with a reversed 50mm lens.

The shots with the reversed lens and tube regularly show a haze, although it is sometimes less obvious.
The reversed lens by itself can be very sharp and clear. I'll put a couple of other images in another post.

My guess is the problem is the extra light bouncing around between the lens and the sensor. With the tube, less of the light transmitted by the lens hits the sensor, ie more bounces around inside. As Nikonian72 pointed out, the reversed lens is not designed for this use and so probably lacks adequate baffles etc, letting in more stray light than the system can handle.

The inside of the extension tube was not completely covered by the absorbant material, I am going to try to cover as much of the surface as possible with either flat black paint or something else to see if that helps.

Summarizing - its not light leak, and its not the reversed lens itself.

Thanks all for the advice!

I'd seen these little "Jumping Bristletails" on the back wall of my garage numerous evenings. Looking for a subject for my weekly blog "HumBugs" hosted by the North Coast Journal (A newspaper local to the Northwest corner of California and surrounding areas) I decided to try to get a few shots up close and personal. This order of insects is one of the most primitive and ancient of all the insects, so I thought they might be a worthwhile subject. I spent a lot of time setting up the shots and getting the lighting close to what I wanted and watching my models saunter off the stage from time to time. What I'm getting at was that I was paying more attention to the camera than the subject. When I uploaded them into my computer I was surprised. They are covered in iridescent scales that change color depending on the angle of the light and viewer. I think we see them as grey because they are primarily nocturnal and the dim light makes everything black and white. Canon 6D, Sigma 150mm macro, Mieke Ring flash, on the first one, and supplemental light from a CAT LED work light.

It has been rainy and windy all day and most of the hood birds and come and gone, in between downpours and brief filtered sunlight. I noticed their food looked low and as I went out to refill, saw this magnificent creature in a tree in the neighbor's yard (~ 40' away). He was kind enough to wait for me to get my gear. Last week, we had a Cooper's hawk who flew in and snatched a sparrow. (He must have been hungry!) Anyway, I don't think this one looks like that one. I am sure folks here know their hawks. Somebody school me, please. Thanks!

https://www.theatlantic.com/photo/2018/10/photographing-the-microscopic-winners-nikon-small-world-2018/572737/?utm_source=flipboard&utm_medium=referral&utm_campaign=promo&utm_content=cm&utm_term=12-2018

This is a link to the Winners of Nikon Small World 2018. It is a macro photography inspiration for me and I thought that it might be for others of like mind as well who would also enjoy seeing some of the fabulous and unique images.

I'm attaching a macro picture of a grasshopper that I took at summer's end simply to provide this link on this forum.