On first load, sx style props in mui is not getting applied in React app

Question

I am trying to customize MUI component like below and importing it other part of my project. This not just happens with input field components, even applying sx on simple component like Box having same problem

<MUISelect
    sx={
        {
          fontSize: "0.8rem",
          p: "0 !important",
          height: "35px",
          "& input": {
            "font-size": "0.7rem",
           },
         }
       }
     label={label}
  />