yeah this seems like something that you in particular (along with a few others) are susceptible to.
there's been a couple of posts about this actually so it could be they are using some sort of new treatment or material.
as the post above mentions, I'd look into a 3rd party genuine leather or other type strap which has worked for you in the past.
as with the washing etc, yes, a good wipe down of the watch and scrub around your wrist is a must (if you aren't already, you'd be surprised how many people don't think about giving the strap and watch a clean)
I have seen various posts over last few years on various sites re rash from watches with silicon straps - including most makes / models - Apple, Samsung etc Many saying it is either sensitive skin or due to sweat / moisture under the band. Have seen the suggestion to lightly dust inside of strap with baby talc ( though not tried this myself as not really had the problem apart from very occasionally) Also wondering if perhaps we sometimes wear the band a bit on the tight side trying to ensure good contact between wrist and back of the watch