....but wait, there is more.
I found that what was tack sharp at 11x14 occasionally had a soft focus spot at 16x20. These were more frequent at 20x24. Where little or no spotting was needed at 11x14, more was needed as size increased.
For me there was nothing wrong with the Mamiya RZ and it’s lenses. Age (72) had made it more difficult to sharply focus the 6x7 image. I shoot 8x10 negatives now focusing with a loupe and rarely have a focus problem at these sizes. A larger negative means fewer enlargements to that size so spotting is also less of a problem.
In 7x17 I only do contact prints. Here it is all about composition. It will be in focus. I rarely spot. You can not crop a contact print. What you print is what you composed.