Synchronization Problem Hammerhead App with Apple Health iOS 26.4 iPhone 12

Since switching from my previous iPhone 15 Pro back to my iPhone 12 the synchronization between Hammerhead Companion and Apple Health won't happen. I tried disconnecting, restarting, everything, but it won't work. Apple Support couldn't help either as they said the problem must be with Hammerhead. In the settings for iCloud Hammerhead doesn't show up. But I believe this to be the usual case as I can't remember seeing Hammerhead there before. Is this a known problem?