Sudden drop in RPM while changing fuel tank

If you are using MOGAS it could be a reid vapor issue. More info here:

I know everyone seems want to blame vapor pressure all the time, but it may not always be true.

This could have been something as simple as it took a couple of seconds for the fuel from switching tanks to pressure up to the fuel pressure from the first tank. It would be worth a few simple test runs in the air and switching tanks to see if the problem persist and does it only happen when there was a long run on one tank then the switch and would it do it again if tanks were switched within 10 seconds of each other? Plus you don't know how his plane is plumbed between tanks and pumps. Different Mfg's do it differently at times. Issues aren't always clear cut without good thorough diagnostics to rule things in or out.

To tag on to what Roger said, I’m curious if Tecnam installs a vapor bypass line as recommended by Rotax for the iS engines. Some manufacturers don’t install this, or at least did not on earlier models. The vapor bypass line essentially short circuits the fuel line loop so that if vapor or air is introduced into the fuel system it can be quickly pushed out the return line back into the tank where it is vented. There is a restrictor in this bypass line so that a small amount of fuel is always bypassed, but it’s inconsequential to the total return flow. Vapor, however, bypasses very quickly.

If this bypass is not installed, then any vapor must go through the injector rails. And because this drops the fuel pressure, the pressure regulator closes down, stopping the return flow, and leaving the injectors as the only escape route for the vapor.  This is especially important if you run a tank dry and gulp some air. Having the vapor bypass line allows the fuel lines to be purged of air very quickly once you switch tanks.  A good way to know if you have this feature is to look at the fuel pressure after you shut the engine off. With the bypass line the pressure drops quickly, without it the lines can remain pressurized for quite some time.  

I have attached the rotax fuel diagram with the vapor bypass highlighted.  With all this said, this surely could be a fuel vapor incident, this is the time of year where it’s warming up and most states still have winter gas. Typically on the iS engines this occurs on the suction side of the pump and produces the symptoms you described.