The vapor pressure of butane at 80F is about 23 psig, pure propane is about 128 psig.
The 30% propane blend has a vapor pressure of around 54 psig at 80F.
Pressure is dependent on the chemical vapor pressure in the cannister, and not on how much is in it. If you increase propane percentage, the pressure will go up, whether the cannister has one gram of liquid in it, or 100 grams of liquid.
This is because when you make a solution of two chemicals that each has their own vapor pressure, they no longer exert it. Their new vapor pressure in this case is approximately equal to their mole fractions in the liquid phase x their pure component vapor pressures. Adding the individual vapor pressures gives the new total vapor pressure. At 99.9 % propane it will be essentially pure propane = 128 psig @ 80F, and at 1% propane it will be essentially pure butane = 23 psig at 80F, and everything in between is ..in between.
At 120 F you are looking at a difference of 110 psig for 30% vs about 240 psig for pure propane.
Not only would cannister safety be a possible issue, but stove operation might change as well due to the higher cannister pressure. The flow of gas could possibly be 40-50% higher at samr settings than for 30% mix. Whether a given stove could adequately compensate for that, is unknown.
Now, unless 30% is an azeotrope (where the vapor has the same conc. as the liquid phase), as liquid fuel is used, the composition in the cannister will change, for better, or for worse, as the more volatile component is preferentially vaporized. Which would create its own set of undesireable issues. If you think about it, would you want a cannister that became pure butane by the time it got to the end? Of course not. Or that the pressure/composition was constantly changing so that the flame wasnt stable for long periods of heating, etc? Nope.