In case of a leak there is a risk in case of a not-ventilated enough underground garage it is possible that the gas accumulates in the lower parts before it can be sensed/sniffed, and a mix of just 2.1% with air is already at risk of explosion if a spark is generated.
Security norms changed in the EU around 2001, the norm is the ECE/ONU 67-01 (though different countries may have implemented differently in the local Law) and LPG powered cars conforming to that standard are allowed (generally) to be parked in underground garages BUT only on the first underground floor and only if the garage is conforming to some (earlier) ventilation standards.
AFAIK cases of explosions/fires related to LPG car tanks are extremely rare (thanks also to the added safety measures mandated by ECE/ONU 67-01), whilst fires/explosions originated by domestic LPG use, while not common, are more common than what they should be (the tanks in themselves are generally safe, but the - often underground - pipings often are not).
Gasoline vapors are also heavier than air, and its lower explosive limit is only 1.4%. If my parked car leaks 10 gallons of gasoline is that really any different? Or are LPG tanks more prone to leaking? I would think not, because they are probably heavier steel not the 18 gauge sheet metal (or plastic) of an automobile gas tank. But if the rule is there, I guess it must be. Hard for me to see the rationale though.
I'm not sure why parking with LPG is a problem but parking with 10-20 gallons of gasoline is not?