Here are two examples of valid f-strings that currently raise SyntaxErrors:

>>> x = 10
>>> f"{x = }"
SyntaxError: invalid syntax
>>> f"{x = :.3f}"
SyntaxError: invalid syntax

Removing the spaces between the curlies is the recommended workaround.

(The examples were found when browsing Python f-strings Are More Powerful Than You Might Think)

These issues should either be documented (in cpydiff) or resolved.

To resolve, see the comment in https://github.com/micropython/micropython/commit/692d36d779192f32371f7f9daa845b566f26968d for background. A solution may be possible by modifying the lexer to strip spaces from the f-string before translating it to str.format.