Again - this is a primary. Explain how you think holding back republican ballots helps democrats in a primary. You do realize that only republican candidates and initiatives are on republican ballots, right?

USPS holding back republican mail-in ballots could sway republican candidates on republican ballots by removing all those votes, so the vote would be weighed more heavily towards in-person voters, but that is assuming an anomaly. That could happen if for example:

Popular candidate A has a scandal during early voting that would probably cause a loss in the General election

Candidate B is not as popular as Candidate A

Most early voters have mailed their ballots before the scandal, and most likely voted for candidate A

In-Person voters know Candidate A is scum so they vote for Candidate B, but there are fewer in-person voters than mail-in voters.

In this case if USPS "lost" most of the republican ballots it is possible that would give the republican win to candidate B, who the democrats likely want because the scandal will hurt the republicans and help their democrat candidate.

This is basically the only way losing mail-in votes could help the dems, and even this very specific scenario is dubious. If they chose the wrong locality they might be losing the wrong ballots. If they lose too many there will be investigations - especially since I believe you can look to see if your vote was tallied in Arizona.

It is a stupid practice, and if I had to guess I'd say it is there purely to help the counties sort the envelopes so they can toss them in the right bins. If they did this in the General then yes - 100% cheat. Primary elections - no.