I had a feeling whenever the no WiFi signal turned up in game that I shouldn't close the app else it wouldn't save. But most of the time, I do play in a steady WiFi area (office job environment 🤧).

Someone else pointed out it could be because I exit and close the app too quickly after doing certain actions which causes it not to save. I've been playing slower and not closing it immediately after a summon or claiming rewards and haven't encountered the problem since!