Would seem mixture went rich (or nearer to correct from previous drastically rich) when it switched to LPG.
Since it is more likely it went rich from correct or a bit lean, first thing first, is mixture correct on petrol?
Assuming it is, go into the map screen, select all the numbers in the table, press enter and apply minus20% to all those figures. Now, when you run autocal again, autocal will begin with a leaner mixture to start with so rpms won't rise as much when it switches to LPG.
Incidentally, if mixture is reading correct on both banks on petrol (lambda flick), sometimes you get a rise in rpm when switched to LPG due to a problem with petrol injector(s), i.e. some flowing more than others so per cylinder mixture isn't correct (only average is correct). If LPG injectors are all good, the increase in rpm can then be due to efficiency gains of the engine when mixture is correct on all cylinders. Particularly sometimes if engine has done a lot of miles on LPG and petrol injectors haven't been used much, or been clogged due to owner running on petrol with next to no fuel in the tank so injectors get fed with mucky tank dregs that made it past the fuel filter..
Simon