I had this rich running issue for a few years, leading to misfires, loss of power, hesitation and death at high revs. Spark plugs completely fouled. I've been to the carb kit route and came back to EFI. Prior to the carb kit, a fault code for the engine temp sensor circuit had started coming up intermittently. I replaced the engine head temp sensor thinking it was the culprit, even though testing of the sensor showed resistance values in spec. But the rich running issue kept happening even after sensor replacement.
Went to two different sets of carbs and couldn't dial them in properly. Went back to EFI and the bike ran well at first and started dying after getting warm or at high revs - spark plugs fouled. Eventually the same fault code for engine temp sensor popped up intermittently.
I knew the sensor was OK. But I fiddled with the connector and realised that if I push the connector on the sensor, I didn't seem to get the fault code whilst stationery. So I unplugged the connector, dismantled it and squeezed the inner pin sockets with a set of pliers for a much more robust connection/mating with the engine head temp. sensor pins.
No more code, no more rich running, misfires and loss of power, fouled plugs. This connector had caused me so much grief and multiple spark plugs and rich running issue over the last few years, even without showing any MIL code at times. But it seems like that was the problem for me.
I guess the issue is that the inadequate mating of the socket on the engine temp sensor pins cause a high resistance connection / open circuit leading to the ECU thinking that the engine is still cold or colder than what it is - which makes the ECU inject more fuel (similar to what the sensor is supposed to do when the engine is cold).
Your previous injector was probably okay. At higher load / revs and heat, the higher resistance in that head temp sensor socket connection might've been causing the ECU to increase the injector pulse width, keeping it on for longer than what's required....(just my speculation based on what I've gone through).