I logged a support ticket about this and they directed me to the dev forums as one of the follow-up resources since I didn't have additional dev support in my plan. 

Since the update, strictly test classes cannot successfully update Product Transfer records. If they try, there is an error like: 
"
Update failed. First exception on row 0 with id 0Lu05000000CakJCAS; first error: DUPLICATE_VALUE, duplicate value found: unknown duplicates value on record with id: 0Co050000008Thu: []
"

Sometimes "unknown" will say "uniqueConstraint". I checked through Workbench, and found that there are no unique requirements on any fields in Product Transfer, Product Item, Product Item Transaction, etc.
Although I can't seem to get Workbench to work with anything 

Turning off all automation that touches these objects does not fix the problem. This affects all of my sandboxes, including the ones I haven't touched in months. 

I can run an exact copy of my test code in Anonymous Apex without issue.

Anyone else having trouble with this?