Why version 5.40 will delete all locally stored photos

I doubt if you would have much luck, but you can try this:

https://helpx.adobe.com/uk/lightroom-classic/kb/recover-lightroom-mobile-photos-itunes-file-sharing....

Alternatively, do you back up to iCloud, and if so, one option could be to wipe your device and restore from a backup previous to the Lightroom 5.4.0 Update, which I think was August 18th?

This though may not be as smooth as it sounds and could lead to other non-Lightroom issues.

Or maybe use a spare device and restore from an old iCloud back up and see if that works?

Do not uninstall the app. The Lightroom team are working on a fix

This is a known issue, please read the guidance given by Rikk on the following thread:

https://feedback.photoshop.com/photoshop_family/topics/lost-my-pictures-and-presets-after-update-today-18th-august-2020?topic-reply-list[settings][filter_by]=all&topic-reply-list[settings][page]=1#topic-reply-list

Hi,

It would help if you could give us some more information.
Does the images reappear after a while?
If you e.g. tap on the cloud icon, does it say syncing?

thanks,
Ignacio