Yesterday, I was editing slides that contained a Bible passage and a heading. Frequently, the preview showed up with blank text. I could get the heading text to display properly by editing the heading, and I could get the passage text to display properly by changing the passage to something else, then changing it back. I later discovered that switching to a different slide and then switching back also had the same effect.
Here's a screenshot:

And here's the log: 8883.636255409572462637_Proclaim.log
The screenshot was taken at 09:55, if you want to match it with the log. However, the issue persisted from approximately 09:50 to 10:00 (when I found the workaround).