New Stickers Disappear Immediately (iOS 18 Messages App)

I've recently upgraded to an iPhone 16 Pro with iOS 18. I have lost the ability to create any new Stickers within the Messages app. When I create a new Sticker, it appears in my list of stickers for ~1 second and then disappears. I've tried restarting my device, logging out/in of iCloud, and turning iMessages off/on. These have not resolved the issue.

Prior to upgrading to the iPhone 16 Pro, Stickers were working as expected on my iPhone 15 Pro. I can create new Stickers on other devices running iOS 18 (e.g., iPad Air). However, nothing I've tried has gotten these working on iPhone 16 Pro.

Any guidance would be much appreciated!