most regulators use a flexible membrane inside (normally rubber/latex) that gets hard and brittle and can crack at liquid CO2 temperatures. also when you inject liquid CO2, which expands to gas at a rate of 1:390, you can get a very large pressure spike in the regulator, which can damage your gauges.
not really "fouling" that would need to be cleaned out, but still not good for the regulator. even if no damage is done, at the very least- you cant trust the gauge readings of a regulator on a sideways tank. that could even be the source of your problems right there.