Sorry for the delay in response.
There isn't any specific preferences setting in the Acrobat Reader DC mobile app which disable or enable the thumbnail view.
However, there could be a possibility that you were viewing the pdf in "Organize Pages". As the pdf shows up in Thumbnail view if you select the Organize Pages tool of the app.
Check once again and confirm, if you are referring to this.
Organize Pages is not available on the iPad Air as far as I can tell.
I just started using Document Cloud and had same problem. I solved it by opening up each PDF that had a standard adobe icon, allowing it to download, scrolled through a few pages, closed document, and the thumbnail was restored.