This not a good rule. Proper PSI is directly related to weight being carried; thus if the weight being carried is 250 pounds running 20 psi under max would be severly under inflated causing snake bite type of flats; whereas carrying weight around 150 pounds would be ok with PSI 20 less then max, and actually better then running at max.

If your running your tires too low on pressure the tube can either start to slowly craw in the tire and valve would move along with it till it is torn, or the lower pressure every time the wheel rotates to the valve at the lowest point it would force the valve firmly into the valve hole in the rim eventually ripping it.

The other weird thing, though I doubt it, is that your using a thin Presta valve in a rim designed for a thicker Schrader valve.

Also check the condition of your rim strip, especially around the stem hole.

Make sure that when you put air in your not flexing the valve back and forth as you pump, this especially happens with hand pumps where one is pumping very hard and rips or weakens the valve area due to wiggling the valve back and forth.

Don't buy cheap tubes.