Is there any situation where iPhone OS will automatically delete messages or attachments to free up space

It's good to be cautious when you suddenly see more free storage on your iPhone than expected, especially if it seems like something may have gone missing. Based on your setup—an iPhone 12 mini running iOS 17.6.1, with locally stored photos and iCloud Messages enabled—it’s likely that your device has offloaded some message attachments to free up space, but hasn’t deleted anything permanently.

When iCloud Messages is turned on, your entire message history, including attachments, is stored in iCloud rather than just on your iPhone. This allows your device to manage storage more efficiently. In situations where your phone is running low on space, iOS may offload large message attachments from the device while keeping the actual data safe in iCloud. The message threads themselves remain visible, and you might see a note in each conversation saying something like “xxx images in iCloud.” This means the attachments are temporarily stored in iCloud and can be re-downloaded as needed.

Since you have your Messages setting configured to save messages forever, and you haven't manually deleted any content, your messages and attachments have not been erased. They’ve simply been removed from the local device storage to create more space. You can confirm this by opening a message thread and scrolling to older content. If the attachments are still in iCloud, they will re-download after a brief moment. You can also tap the “info” icon in a message thread to view photos and other media that may need to be fetched from iCloud.

If you want extra reassurance, you can verify that your messages are safely stored in iCloud by checking from another Apple device that uses the same Apple ID. Open the same message threads there and confirm that the attachments are still present. You can also check your iCloud storage by going to Settings > [Your Name] > iCloud > Manage Account Storage > Messages. This will show how much space your messages are using in iCloud, which reflects the full content that’s stored.

In summary, your iPhone hasn’t permanently deleted any messages or attachments. It’s simply offloaded them from local storage due to low space, while keeping everything safe in iCloud. This is normal behavior when iCloud Messages is enabled, and there’s no need to worry unless you manually deleted something yourself.