In years gone by, whenever an application had a problem, you could always count on certain steps from tech support (the three Rs)
1. Reindex (if a database application)
But I've been spoiled in recent years by the iPhone where this seemed to be a thing of the past. This might be because most of the data is on the cloud and so it was only ever syncing. The phone was just a "vehicle" while all the data was online. Sadly, when I upgraded to the new iPhone XS this fall, I found a few apps that didn't work right.
Since a number of application developers haven't kept up with the latest iOS phones, I attributed much of it to apps not being up to snuff but some were newer apps and they didn't work at all. Security applications wouldn't register a login, apps would just quit for no reason but worse, my favourite Health tracking app, Pillow, seemed to have lost all of my older data.
It was still in Apple Health but Pillow, which tracks a lot of other useful information about sleep, had seemingly lost everything else.
(Pillow is an awesome app either with or without the Apple Watch)
After contacting tech support, I had the answer.
Delete and reinstall.
To be sure, don't delete the app and let it remove your data but remove the App and just reinstall it from the App Store.
The problem is related to sync - "There are some occasions where the iCloud container does not get initialized properly and re-installing the app usually solves the problem. "
So while we may no longer need the first R, it looks like the last two are still needed.