Has anyone ever noticed any variations in the thickness of IS to Post mount brake adapters. I’m having slight clearance issues on what seems to be a thicker mount.

Yep. Its a standard just not that standard. Usually fixed with a couple of washers but sometimes you just need another brand of mount.

My problem is the other way, I need a thinner mount. I’ve never had issues with Shimano mounts, but the current one I’m using (brand unknown) is a bit thicker.

I’ve encountered the same problem. There’s a couple of adapters in my spares box that are too wide and rub on the disc.

shimano front and rear IS ones are different too with one having slightly off-centre caliper holes so the trick of a 160mm front being used as a 180mm rear doesnt work.