Do you have any aggressive battery saver app running?
What notifications are they? Are they random notifications ? Or specific ones?
So this JUST started happening all of a sundden on BOTH devices? So do the same notifications disappear on both devices or are they random between both?
Although difficult to diagnose over the internet, it sounds like something local to your device and how you have things setup.
If the phone works in every other way (makes calls, sends/receives texts/mms, uses data properly), its most likely something local to your device, so re check your settings and app permissions.