When using the “future due” graphic on the stats screen and selecting “all”, the final card isn’t shown:
Anki instead shows the entire time period (including “today”, I think that is the issue) up to +239, whereas the card has a due of 240.
Part of this might be the graphic binning 5-day intervals, where 240-244 is always an interval…
This issue happens both with the subdeck with the due 240 card (smallest due: 3) and its parent deck (smallest due: 0).

The card might have an interval of 240 days, but when was its latest review? Maybe it was some time ago. The due date will be calculated as the latest review date plus the interval.

What does the Card Info show for that card? It will have Latest Review, Due, and Interval fields.

I accidentally typed ivl where I meant due, sorry (misremembered what they stand for).
That card is due in 240 days, but it has an interval of 8.03 months (and the latest review was yesterday).

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.]

I just checked this in 23.12.1 and it’s still happening. And it’s not for just one card, it’s for however many cards are due on the latest date that any cards are due.

I have 800 as the Maximum Interval limit in deck options.

In everything below, I unchecked the Backlog checkbox in the Future Due graph to simplify things.

When I search deck:X prop:due>=0 prop:due<10
the number of cards selected in the Browse window matches the mouseover “cards due” count for the first (leftmost) histogram bar of the Future Due graph in the Stats window.

And when I search deck:X prop:due>=0 prop:due<800
the number of cards selected in the Browse window matches the mouseover “Running total” for the last (rightmost) histogram bar of the Future Due graph in the Stats window.

But when I do deck:X prop:due=800
then there’s a lot more than just one card.

In fact, that includes all of the cards that I reviewed today that would have had due dates many years in the future if there was no Maximum Interval limit. So in fact deck:X prop:due=800 rated:1 returns the same cards as the same query without the rated:1.

I’m using FSRS, but I don’t think that makes any difference, since you saw it in pre-FSRS version 2.1.65

2.1.65 is pre-built-in-FSRS, but out here on the fringe, I’m using custom FSRS just fine. But, I can confirm, none of these 4 furthest cards have been scheduled/rescheduled by FSRS.