https://drafts.csswg.org/css-text-3/#cursive-tracking

“if the UA cannot expand text from a cursive script without breaking its cursive connections, it must not apply spacing between any pair of that script’s typographic letter units at all (effectively treating each word as a single typographic letter unit for the purpose of letter-spacing).”

Devanagari isn't "cursive" but it is "connected." It should probably behave like the above quote describes. Therefore, the spec should probably switch the word "cursive" with the word "connected."