I had this same issue and it took me a week to figure it out. I hadn't updated my existing sticker packs in 2 years and decided to do so a couple of weeks ago. When I updated Xcode and opened my older projects, it wanted to update them, so I naturally let it do so. Unfortunately, it updated my projects to have a deployment target of iOS 12. But sticker packs apparently need to be set to iOS 10. I do not know why this is. I think it's because while everything else in the MacOS universe is 64 bit, the sticker packs need to be backwards compatible for older phones where they are in older versions of iOS that support it. My guess.
Once these projects got updated, even rolling them back to old iOS versions didn't help - so what I did was create a whole new sticker pack w/ the same name, made sure the target was iOS 10, put in my assets, updated the build number and boom - it was accepted by Apple when I uploaded it. :)
This was before the silicon announcement. This resolved the 32bit issue. BUT - now I can't update other sticker packs due to the silicon issue.
