I’m going to do a few tests tonight but to me it looks like that if you’re reduced feed rate is more than half the total length of your standard feed rate it just post-processes the program speed as 0
I’m not sure why this is happening it is pretty rare to have more than 50% of your program length feed rate optimized but it should be able to process it regardless. Once we can actually define this issue I think it needs to be addressed maybe with a updated Fusion 360 post processor?