Code point sequence for UM doesn't produce ligature

[r12a]

Font

Noto Serif Ahom

Where the font came from, and when

https://notofonts.github.io/

Font Version

2.007

OS name and version

macOS Sonoma 14.7

Application name and version

Web-based picker (in devt) https://r12a.github.io/pickers/ahom-aho/index.html?text=%F0%91%9C%92%F0%91%9C%AA%F0%91%9C%A2%F0%91%9C%A4%20%20%F0%91%9C%92%F0%91%9C%A2%F0%91%9C%AA%F0%91%9C%A4%20%20%F0%91%9C%92%F0%91%9C%A2%F0%91%9C%A4%F0%91%9C%AA

Issue

In order to achieve the ligature of the i and u, which i believe is expected, when a nasal coda follows, it is necessary to use a non-standard ordering of the vowel sign code points (the first example).

Normally Ahom expects the above-base vowel sign(s) to precede the below-base vowel sign. Then it seems appropriate (and it works in the font for other composite vowel signs) to store the i before the (a)m vowel sign, since that is the spoken order, and then follow with the u. If you do that, however, the ligature breaks (the second example).

It's necessary at the moment to reverse the order of the i and the am to produce the ligation (the first example).

The third example shows (expected) behaviour if the below-base vowel sign is stored between the i and the am.

I suspect that the code point sequence in the second example should produce displayed text like that of the first example.

Character data

See the picker link. Select characters and click on List Characters to see code points.

𑜒 U+11712 AHOM LETTER A
𑜪 U+1172A AHOM VOWEL SIGN AM​
𑜢 U+11722 AHOM VOWEL SIGN I​
𑜤 U+11724 AHOM VOWEL SIGN U​

𑜒 U+11712 AHOM LETTER A
𑜢 U+11722 AHOM VOWEL SIGN I​
𑜪 U+1172A AHOM VOWEL SIGN AM​
𑜤 U+11724 AHOM VOWEL SIGN U​

𑜒 U+11712 AHOM LETTER A
𑜢 U+11722 AHOM VOWEL SIGN I​
𑜤 U+11724 AHOM VOWEL SIGN U​
𑜪 U+1172A AHOM VOWEL SIGN AM​

Screenshot