I’m able to reproduce that, I think. (If it matters, this is in 2.1.65 QT6, Windows 11.)
The graph for Future Due (all) tells me that there are 4 cards in the final band.
Clicking on the bar brings up the Browse window with a corresponding search of deck:current "prop:due>=1340" AND "prop:due<=1353", but the results show only 3 cards.
The 4th (and furthest in the future) card is actually at prop:due=1354.
I note that the rest of the graph is divided into 20-day bands, so that final band being only 14 days appears to be specifically set for 1 less than the 4th card. The band would only need to go to 1348 to capture the other 3 cards, so it seems to not be cutting off the furthest card from the results, just cutting the furthest day – if that distinction makes sense.
[Even though I clicked a few things to look at this, it was mostly to satisfy my own curiosity. 
I’m not sure there would be much utility in fixing this for 1 card in the graph – unless there were other ramifications.]