The installer turned up pressure, so we can assume this is a pulsed LPG injection system (VSI etc), not a Prins mixer system...
In which case problems are very unlikely air filter related.
Points to check: pressure, LPG rpm reading, reducer temp sensor (also covering coolant flow problems through the reducer), solenoids, ECU working properly, ECU settings, LPG Injector driver unit, vehicle lambda probes, ignition system, other electrical problems. Given the nature of the problem, a diagnosis should involve a test drive with laptop connected to the Prins system and OBD scanner plugged in.
Prins reducers are seldom vacuum referenced (output pressure isn't relative to manifold pressure), so reducer absolute output pressure doesn't change much as engine load changes. Seems the installer found pressure was too low - Implying he will have looked at the working and minimum pressure settings in software and found actual reducer pressure to be lower under some conditions than one of these software settings to the point where the ECU generates a low pressure error or believes you have run out of gas, either situation will cause a switch back to petrol. This is a common thing to see on older Prins installs (the spring which controls the pressure goes soft over time) but is usually easily sorted by turning up reducer pressure (increasing pressure on the spring). Some installers cheat, don't even go into software but just guestimate the problem and turn up the pressure a bit.
As an aside, I see many Prins systems setup to expect a certain pressure (software settings) when they would have been better setup to use a different pressure and software configuration - Possible that a check/adjustment of calibration could cure all your problems and leave you with a better calibrated/setup system for just the price of a diagnostic/simple tune up, i.e. maybe calibration should be changed rather than pressure, or both should be changed. At least this should be checked.
Reducers can also fail in a way where pressure increases when no gas is being used, the ECU then interprets an over-pressure fault condition, more likely to occur on engine over-run (when no gas is being used) and with much the same result i.e. causing a switch back to petrol.
Don't know who the installer was, I probably wouldn't comment on an individual firm unless they were well known in the industry as rip-off merchants (there are a couple in this area), but as a general comment I don't have much faith in the abilities or intentions of a lot of Prins specialists while at the same time many other installers don't have much knowledge on Prins but some would still take your money.
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