Did you thoroughly check that you downloaded the correct app from Apple store? While it's rare, sometimes malicious apps can sneak into Apple store, too. Apple store verifications are not without errors and oversights.

How did you enter your wallet recovery words on your iPhone? Carefully typing by hand (assuming the keyboard app is safe) from an analog source or did you use some copy/paste actions from a digital source?

There's a lot that can go wrong if you're not aware of pitfalls and your descriptions lacks quite some details to judge what might have gone wrong. To be clear: I don't want to defend Atomic Wallet in any way. They brushed away valid security concerns of their product from past audit(s) without addressing them properly. I'm not convinced that Atomic Wallet knows what they're doing with their complex product.