There’s a few things that can caused the ATC to cancel IFR:
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Your transponder is malfunctions, if you spawn into the flight mid-air, or on the runway, sometimes you never go through the entire IFR clearance process which makes your transponder to use the default squawk code. The longer you’re transmitting this incorrect default code, the more the ATC would get upset, and until you correct it, the ATC will cancel your IFR. Best solution is, either you start your whole flight from cold and dark, or that you’re standing by your transponder to reset into the assigned squawk code as soon as you get the message.
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You fail to respond to the ATC instructions a few times in a row. Sometimes you missed it, sometimes the game bugged that you can’t really respond to anything. Either way, without responding, they would eventually cancel your IFR.
Actually, keeping the AI Copilot to handle ATC on is actually helping you to prevent the IFR from being cancelled since they will keep responding to the ATC calls. So if you get cancelled while having the AI handling it, chances are it’s the first cause.