For example: https://app.everviz.com/edit/535863?editor&panel=chart_customize.
It seems to possibly have something to do with screen size or resolution. But strangely in the example 980 is not shown, but 933 is. This would still take up the same amount of space.
Even a downloaded chart is sometimes missing some numbers from the data labels
Yes you are correct the width of the screen is the deal here. While it seems to you (and me) that a label should fit, the rules for determining for whether one should be shown is much simpler than the inner workings of our brains.
As of today, we have no recommended way of enforcing labels to display in more cases other than increasing the width. You can force it to show, surely, but that is generally not advisable for several reasons.