Hi Icollins,
I had also met this problem.
When I set the MUX output is N couter output and it does't work as well (R counter/ VDD are well).
As your mention as above "Some old ADF4350 parts had an issue with the N counter." and how can I know what the date code has been solved this problem?
PS: I also had tested the ADF4351 EVB and it has the same issue.