Default [Q] Multi-Window Tray disappearing and reappearing Apps

All,

I've got a VZW Galaxy S4 that is stock and unrooted. For the most part it works just fine. But on occasion there are a few apps that will keep appearing then disappearing from the MW tray.

The biggest ones that I use frequently and have them on the tray are Moon reader and Reddit News Free. Frequently they will show up in the tray, but then vanish without a trace from the tray. If I go to edit the tray they are not there either. The apps are still installed and I can get to them through the app drawer or any other shortcut.

Then when one or the other updates via the play store both will be back in the traY. I can't figure this out.

Has this happened to anyone else? FWIW this does not happen with my Tab 3 8.0.