The ballot drop boxes in Washington and Oregon both have fire suppression systems that are designed to activate when the temperature inside reaches a certain point, coating ballots inside with a fire-suppressing powder.
For unknown reasons, the system failed to prevent the destruction of hundreds of ballots in Vancouver, just across the Columbia River from Portland.
While this is a shit move by whoever is doing this, it makes virtually no difference for the presidential election. WA electoral votes are winner-takes-all, and always go Dem. A few burned boxes won’t come close in making a difference for president, Biden won by almost a million votes in the last one. It will probably have a decent effect on local elections though.