Achoo the mulch I used was tip mulch heated treated in large piles. I have used wooden sleepers in clay for years and they didn't rot like the ones with mulch against them. Not same timber but I've seen hardwood fence posts last for 50 years before rotting off in water soaked clay holes. I know because I pulled down a fence this year and replaced it. So I have to disagree as I'm going by experience not hear say.
Tip mulch is just cheap $hit recycled rubbish heated out in the open full of bacteria especially in winter, no wonder. You added further about hardwood which has no comparison with treated softwood sleepers or posts or mulch without any actual reasons or science. I wouldn’t say 20 years experience working in the Horticulture/Landscape industry and six long years study is hear say, you might, so I agree to disagree.