I will probably give it to you that Iranians perceive that Europeans can offer better products or services. But the reality is that it is Iranians' wishful thinking. Between Iran and GCC/Arabs, I don't think EU would treat Iran any better. And between Iran and Israel, US would dump Iran in a heart beat. China does not have any design other than that Iran is a geopolitically important county along the BRI. But when push comes to shove, if China sides with GCC/Arabs, Iran would definitely be on the losing side. And Iranians are always 2nd class in the eyes of Europeans at best. So, I guess it is the perception that drives people's pockets again.Western civilian aircraft are simply better, it wouldn't be very rational to buy Russian planes when better options are available. Especially when you're trying to sell your oil to the West.
Iran also feels betrayed by its partnerships with China because many companies withdrew because of American sanctions. The American market was always more important to China than the Iranian market, which is very rational.
Despite occasional commercial disagreements, the Iranian government knows better than to rely on the West. Who knows when the next Trump will be elected? Politically Iran will always be on the anti Western side. A strong Iran is worth a lot more to China than today's weak Iran. That means sharing the Iranian market with international brands. Chinese products still have quite a poor reputation abroad, but just like Made in Japan used to stand for cheap knock offs, Made in China will soon be known for the quality. Once the new deal gets signed, Iran will probably buy new planes from Airbus. But if the C919 proves itself as a reliable plane, Iran may buy from COMAC 10 years from now. They're already buying Chinese cars or building them under license.