Why is the Shimano R3030 FD not compatible with the 3503 FC? I have an older version of the Triban 520 with the Shimano Sora 3503 groupset (3x9 speed). I neglected it for two weeks over winter and the salt has done some damage. The spring in the front derailleur no longer returns the cage. I am looking at replacing the front mech. It seems much easier to find a R3030 front derailleur rather than the older 3503 front derailleur.

From the Shimano compatibility documentation I can see that the the 3503 shifters are compatible with the R3030, 3503 and R353 front derailleur. Though, it seems that the R3030 front derailleur is not compatible with 3503 crankset. Can anyone explain why this is the case and suggest a suitable replacement?
This is from a long time ago, but for the sake of others, I believe it's because of the cage size, I was trying to do the opposite with an r3030 crank and 3503, and I was not able to get it to shift smoothly until I paired the r3030 crank with r3030 front derailleur.