I just had the same problem some minutes ago. I had just finished setting up a relation between two Array columns in different tabs and a Lookup column to retrieve data through that relation. It worked like a charm for some time until my app stopped working and all the columns containing data were empty.
In case this can help, a platform update took place more or less at the same time everything stopped working. As a result of the update, among other things, the “upgrade to pro” and “share app” buttons had been relocated to a different place.
After trying to solve the problem unsuccessfully, I decided to build a simple app from scratch, with nothing more than an array relation between two basic tables, but it did not work either.
I hope this is just a bug and everything goes back to normal soon. Setting up array relations is a very useful resource that makes GlideApps very powerful!