We see that Vowel signs attach to Consonants easily.
We have added a new character U+097A and made the new glyph heavyya-deva.
When we type this new glyph, then existing Vowel signs do not attach to it.
How to achieve this attachment?
It might be best if people with more knowledge about Indic fonts provide answers.
In case you haven’t looked into it yet, here is a document that explains how Devanagari Script is processed by a shaping engine:
https://docs.microsoft.com/en-us/typography/script-development/devanagari
We found the solution.
This works in Word 2016 or later editions.
Also works in Libre Office 6.4.6.2 with the Sanskrit Language pack.
