Thanks for the repro @ImanMahmoudinasab

Seems like the margin is coming from the InputAdornment styles:

  /* Styles applied to the root element if `variant="filled"`. */
  filled: {
    '&$positionStart:not($hiddenLabel)': {
      marginTop: 16,
    },
  },

so we need to basically remove the margin. I would propose the following change:

diff --git a/packages/material-ui-lab/src/Autocomplete/Autocomplete.js b/packages/material-ui-lab/src/Autocomplete/Autocomplete.js
index 88e164428..77c4d7c34 100644
--- a/packages/material-ui-lab/src/Autocomplete/Autocomplete.js
+++ b/packages/material-ui-lab/src/Autocomplete/Autocomplete.js
@@ -122,6 +122,9 @@ export const styles = (theme) => ({
         padding: '4.5px 4px',
       },
     },
+    '& [class*="MuiInputAdornment-filled"][class*="MuiInputAdornment-positionStart"]:not([class*="MuiInputAdornment-hiddenLabel"])': {
+      marginTop: 0,
+    },
   },
   /* Styles applied to the input element. */
   input: {

This is the end result:

What do you think about this? Would you like to work on a PR for this? :)

@mnajdova +1

The structural difference is that the OutlinedInput has the <input /> takes the whole height, while with the Autocomplete, it behaves likes a multiline OutlinedInput, the <input /> only takes the minimum amount of height it needs to function correctly.

We have the same issue with:

<TextField
  InputProps={{
    startAdornment: <InputAdornment position="start">Which?</InputAdornment>
  }}
  multiline
  label="Combo box"
  variant="filled"
/>

https://codesandbox.io/s/material-demo-forked-1bdng?file=/demo.js

This has the same root cause than what we have with #21409 (Autocomplete) and #22241 (multiline TextField).

Maybe in the future, we should look into how we can unify the CSS for multiline and Autocomplete use cases.