LINE chat history is backed up to iCloud, but LINE says no chat history backup found

My partner just switched to a new iPhone, but there was a problem, so she switched back to her old iPhone. LINE worked fine after switching to the new device, but when she switched back, LINE saw multiple devices trying to use the same account, and deleted all chat history on both devices.

Fortunately, she had LINE iCloud chat history backup on (it's 5.2GB in iCloud). So we tried to install LINE fresh and restore from the iCloud backup. Her iCloud account that the phone is logged into and contains her LINE backup uses the same email address as her LINE account. But for some reason, LINE doesn't see the iCloud backup on the setup screen. There's no backup detailed listed, and it reports an error "No chat history found" or something similar when she tries to restore it. Question 1 is how can we make LINE find the iCloud chat history backup?

As a troubleshooting step, we tried restoring both devices themselves using iOS iCloud backup. When she opens LINE after the backup, her old chats briefly appear grayed out while the app is initially loading, but then it says the LINE account is being used on another device, and says all the data must be deleted. We then go through the steps to login to the account, but all the chats are gone since it can't find the chat history. We then tried turning on the 36 hour transfer account setting and going through the device reseting and restoring from iCloud process again, but we get the same result. Question 2: is there any way to make the LINE app realize this isn't a different device, it's just been restored, so it doesn't get confused and require the restored account data to be deleted?

Any other tips on how to recover from this situation?

Thanks so much!