My minuscule pet peeve is that having only one source where the number 5 is depicted with a triangle (all others show it as a separated segment, like the number 6 but shorter), that's how every article or library draws it. It's all because the guy who wrote a book about them saw that source first so he based his figures on it.
Here's a small summary about the numbers with many examples: https://www.unicode.org/L2/L2020/20290-cistercian-digits.pdf
Being first matters :')
I wrote a font for these, which does use the triangle-5 and the vertical layout: https://bobbiec.github.io/cistercian-font.html (recent discussion here: https://news.ycombinator.com/item?id=46939312)
And my associated writeup: https://digitalseams.com/blog/making-a-font-with-9999-ligatu... .
As mentioned in the blog, I think the horizontal layout makes more sense too (in terms of writing order). But just like the triangle-5, the vertical layout is more commonly seen, so that's what I stuck with.
It might not be accurate but it does seem like it'd be easy to mistake a 5 and 6 without the triangle. Especially when the characters are being hurriedly written by hand. If I were going to use this system, I'd be sticking with the triangle.
It would never have occurred to me that anyone would want to get these into a Unicode standard. This document you linked is excellent, thank you.