Hot take: quit backfilling post holes with the dirt you dug out
I dug 40 holes in rocky clay for a split rail job outside Bend last month and backfilled the first 10 with the native dirt, then a guy named Dale told me he always hauls in 3/4 minus because the clay turns to soup after one rain and pulls your posts, and sure enough two of mine were loose by week two, so what are you all using for backfill when the ground is junk?
This is like when I tried fixing my own leaky faucet with the same old washer and it just leaked again a week later. Sometimes the stuff that came out is exactly what you shouldn't put back in. For junk ground I've had good luck with clean gravel, lets the water drain instead of turning to mush.