Your data may be lost if you did not make a backup of iOS 18.5 before you became a beta tester. Using iCloud backups your previous backup is overwritten with the next backup. When you downgraded it may be the case that your backup was overwritten with your now empty iOS 18.5 device. To remove any beta, it requires your device to be wiped with a computer and any data restored would need to be from that 18.5 backup. The backup using the beta is useless when restoring to iOS 18.5. The best way to perform a backup before using any beta software is by backing it up to a computer. That way the data is not overwritten and you will have the option of which backup you want to use.
Before deciding to become a Beta tester, you need to be aware of what you are doing.
Always back up your iPhone, iPad or Mac before installing the beta. Since Apple TV, HomePod mini, and HomePod (2nd generation) data and purchases are stored in the cloud, there’s no need to back up your devices. To use a previously-released version of iOS, iPadOS, or macOS, you can restore your device from the backup that you created before installing the beta. Please note Apple Watch cannot be restored to previously-released OS versions once the beta is installed.
Apple Beta — FAQ