Here is their FAQ:

I think some of my Hotels.com rewards are missing: One Key is currently only available in the US.

• If you have created your account on the US site and have current rewards earned on Hotels.com outside of the US, your confirmed Hotels.com Rewards stamps and reward nights will be converted to OneKeyCash at their current value. We will award 10% of the value of each Hotels.com Rewards stamp collected in OneKeyCash. Your pending Hotels.com Rewards stamps will remain on your Hotels.com Rewards account and can be viewed on non-US Hotels.com sites.
  
• If you have created your Hotels.com Rewards account outside of the US and your current rewards were earned outside the US or on multiple sites including the US, we have not moved your Hotels.com rewards to One Key. Travelers who visit the US site and log in will be enrolled into One Key. You’ll have a One Key rewards account on the US site, but your Hotels.com Rewards reward nights and stamps will remain on the non-US site where you created your Hotels.com Rewards account. To use these Hotels.com rewards, you’ll need to log into your non-US Hotels.com Rewards account. One Key will be coming to additional markets starting in 2024.
  
• If you created your Hotels.com Rewards account outside the US but your rewards balance is from bookings made only in the US, then your account and rewards have been migrated to your US One Key account at their current value.
  

Also, non-US Hotels.com websites should still earn stamps for non-US Hotels.com Rewards accounts, considering the following disclaimer I received when switching regions on the website: "One Key™ is our new loyalty program, currently only available in the US. If you're already a member of a loyalty program and want to continue enjoying its benefits, don't change your region or country."