marco_py Frontend sends JSON, gets 422. Postman with the same body: 200.
Compared the bodies byte by byte in the network tab. Identical.
Identical bodies, different headers. Your frontend sends Content-Type: text/plain (fetch does that for string bodies without a header), so FastAPI does not parse JSON. Add headers: { 'Content-Type': 'application/json' }. The 422 body tells you: value is not a valid dict.
marco_py 
For the next 422: log what FastAPI actually rejected. The default response body is easy to miss in the network tab, a handler puts it in your server log:
@app.exception_handler(RequestValidationError)
async def log_422(request, exc):
logger.warning("422 on %s: %s", request.url.path, exc.errors())
return JSONResponse(status_code=422, content={"detail": exc.errors()})The loc field says exactly which part failed: ["body"] for the whole body means it wasn't parsed as JSON at all.
marco_py