One question to consider is whether you are failing to receive flow exception emails for ALL process builders or if the exception emails are just failing for one particular process builder.

I just had a process builder fail.  I received an error message in the UI that read:  

"We can't save this record because the (Object__c) process failed. Give your Salesforce admin these details."

There were no details, and though I recreated the error, checked deliverability in my sandbox, made sure I was on the Apex Exception Email list, etc., I received no exception email.

I intentionally created an error for a different object, however, and received the flow exception email like I always do.

I just realized that for the object whose process builder is failing, I just changed a lookup relationship to a master/detail relationship, and this particular process builder (built by someone else on my team) has tons of references to the record owner ... Record owners don't exist on this object anymore, since it's now the child in a master/detail relationship:

https://salesforce.stackexchange.com/questions/162147/can-i-feel-safe-to-change-a-lookup-relationship-field-into-master-detail-relatio

I don't know why an exception email can't be generated in this scenario, but this appears to be my problem. I discovered it when I attempted to clone the process builder with the recent relationship change.  Again, other process builders seem to be generating exception emails without issues.