I did of course already see the cite previously provided by @Joey_P. I don’t know enough about the stunt to know if there was any change to the vote count as a result of the stunt. Based on what has been provided, it doesn’t appear the ballots were tampered with or uncounted. So…
With respect to your Humboldt County cite, that had nothing to do with drop box tampering. It was simply an oversight on the part of the elections staff.
From your cite:
The Humboldt County Office of Elections has determined that this error occurred when election workers checked the drop box and there was a miscommunication about whether it had been fully emptied. The Elections Office has already updated its protocols to make sure this will not happen again. A lock out, tag out procedure is now in place for every drop box prior to certification to ensure each one is physically verified as empty and secured before elections results are finalized.
In your LA cite:
AIR7 video captured video of the ballot box, which is located near the Civic Center in Downtown L.A. Election officials discovered a small number of vote-by-mail ballots with fire damage in the drop box on Sunday morning and filed a report with the Los Angeles Police Department.
(Emphasis mine.)
The remedy here is for the elections staff to contact the persons whose ballots were damaged and have them resubmit their ballots. Kind of a pain, but not earth shattering, and won’t change the outcome of an election.
With respect to your Washington State cite, that’s the one I was thinking of when I said I was pretty sure it was one in Oregon. I remember those instances well, because one was in Oregon where little damage was inflicted due to remedial measures being taken to douse a fire started in the box; the other was the same night in Vancouver, Washington, which I’m sure you know is just over the river from Portland. That one was marginally more successful because Washington State had not anticipated fire damage the way Oregon did. Looks like your cite actually indicates that Washington took steps to protect the boxes better – as they should have.
In neither case were the damaged ballots enough to flip the results of any candidate’s election.
This stuff happens in every election.
Glad you posted the NPR cite. As stated, drop boxes are pretty darned reliable.
Overall, I don’t think ballot drop box tampering is nearly as ubiquitous as people believe. It’s scaremongering, and I hate that.
You haven’t met my mail carrier. 
I don’t trust him with the day-to-day mail, let alone a ballot!
I also do what @Joey_P does: I verify my ballot has been received and counted by our elections officials. I do it for every election. My drop box hasn’t let me down yet, and I’ve been using it for 21 years.