Typed Devanagari not consistent with native macOS app

With the standard macOS "Devanagari QWERTY" input method, typing the keys "s f v" in a normal Mac app like TextEdit yields "स्व". Typing those same characters in a QLineEdit or in WebEngine yields the following instead: "स््व". The same issue exists when using the non-QWERTY Devanagari keyboard layout as well. Chrome appears to exhibit this same behaviour, so I presume it's an issue in an underlying library.

The user who reported the issue to me included some more information in their report:

"I'm using the Devanagari - QWERTY keyboard mapping that comes as part of macOS.
In Roman characters I'm trying to write svāmin.
Devanagari "characters" are syllables with a default vowel a, so that the keyboard actually represents things like sa and va.
To get rid of that first a and arrive at sva, I need to hit the f-key (the actual letter on the keyboard, not a function key). As I hit that key, a small slash ् will appear below the Devanagari character, saying the a is missing.
They next character I write should then be "joined" to that character.
Instead of सव (sava) I should see स्व (sva).
But what I get in Anki is स् ् व, which might be something like s.-va in Roman characters.
So that you can reproduce exactly what I'm doing within Anki, here's my sequence of steps:

switch to the Devanagari - QWERTY keyboard;
hit the s-key: स
hit the f-key: स्
hit the v-key: स्व
This is where one error occurs: the two characters do not appear joined in this way.
hit the a-key: स्वा
hit the m-key: स्वाम
hit the i-key: स्वामि
hit the n-key: स्वामिन
hit the f-key: स्वामिन्
hit the [space]-bar: स्वामिन्
This is where another error appears (after hitting the [space]-bar): I get an additional ् character at the end."