I think the fading in and out has to do with the duration of the grain.
The sample is always playing back using grains and they have a certain duration.
Even at the longest duration setting, it is still a grain (slice) that has a set length.
So it fades in and out at the beginning and end of the grain, no way around that.
Adding another grain stream with the right offset can hide the fades to some extent.
That is my guess anyway…