I have four addresses in my wallet, of which one is watch-only.
In the main view the second address (not the watch-only address) shows a very, very small QR code unlike the others. Also if I tap that second address it does not show the dimming QR code like the others. Instead it shows the Android "Share with..." dialog.
My guess is that you have a long label for that address. If the display is small enough and the label long enough, then the QR code will shrink to make the label fit, and eventually disappear entirely if the label is too long. Does this fit with your observations?