The un-eveness looks like what is referred to as bromide drag. But I do not think a developer must contain bromide to exhibit this effect. It is typically an agitation problem. Stand development frequently exhibits this phenomenon. I do wonder why you are using Diafine. I've always considered that a mushy developer, used when speed is more important than gradation and edge sharpness (in this case, low acutance). If you want to increase shadow density without edge sharpness loss, you may want to consider D-23. Although you have to mix that yourself using a scale, or you can look up teaspoon equivalents. It's very inexpensive and a classic. I have used it with TMY often. A lot of people think of D-23 as a low contrast developer because it's excellent for compactions. But it has plenty of snap when normal developing times are used. Of course you may have perfectly good reasons for using Diafine that I'm unaware of. I guess I have never encountered anyone using it for large format. Agitate more frequently, and invert the tank if possible.
Best of luck,
Doug