Typst 0.14 was supposed to fix the long-standing issue that $f_i(x) renders the (x) in the subscript. The relevant change was made in Require parentheses in all function-like sub/superscripts by mkorje · Pull Request #6442 · typst/typst · GitHub after lengthy considerations and discussion spread across Discord and here in this thread.
However, a new case that hasn’t previously been considered and whose behavior has regressed with #6442 has now come to light in discussions on Discord (specifically the parsing precedence in $ 1 / f_a(x) $).
There is a potential fix for this issue in Math mode implicit function call update and parser refactor by isuffix · Pull Request #7072 · typst/typst · GitHub. However, that PR only revealed deeper issues with the design and we’re not confident that it is the best we can do for Typst.
As we don’t want to put the burden on users to migrate their documents twice, we have thus decided to revert and postpone the change. That said, we are still committed to fixing the rendering of $f_i(x)$.
In particular, we are planning to prioritize a more holistical reconsideration of all edge cases and issues of the current math syntax, so that we can make one confident change rather than spread-out, disruptive fixes.
The revert is being made in Revert math precedence change by laurmaedje · Pull Request #7084 · typst/typst · GitHub.