[Date Picker] DatePicker improperly adds extra 0's on Single-Digit Months and Days; It Should Strictly Respect Given Date Format

Steps to reproduce

Link to live examples:

Steps (in live example):

Set DatePicker.format to:
1. M/D/YY if using dayjs
2. M/d/yy if using date-fns
In the date picker, select a date with a single digit month and day
Compare the DatePicker's TextField's date format to the library's format(date, fmt) return string (see Demo.tsx)

Current behavior

V5

Expected Behavior Happens

V7 - Bug Occurs

The DatePicker's TextField displays adds 1 extra 0 to single date months and days despite the date format specifically wanting single-digit months and days.

const fmt = "M/D/YY";

export default function FirstComponent() {
  const [date, setDate] = React.useState(dayjs());
  return (
    <LocalizationProvider dateAdapter={AdapterDayjs}>
      <DatePicker
        format={fmt}
        value={date}
        onChange={(newDate) => {
          setDate(newDate);
        }}
        slotProps={{
          toolbar: {
            toolbarFormat: fmt,
          },
        }}
      />
      <Box>{date?.format(fmt) ?? "Please select date"}</Box>
      <Box>
        If formatted date is not the same, there is a bug in the DatePicker.
      </Box>
    </LocalizationProvider>
  );

Expected behavior

The given format to the TextField should be the same as the library's format(date, fmt) call. This was the case in v5 of the Date Picker library.

Context

In most cases of our frontend (not all), we expected all our date formats to follow the following pattern:

single digit months should just display the month number with no extra zeroes
single digit days should just display the day number with no extra zeroes

We're doing this for mostly... reasons (keeps the text field compact etc.).

When updating from v5 to v7 (skipped v6), we noticed our date pickers have added zeroes to single digit days or months regardless of the given date format. The format seems to be followed as expected except for the single digit case I can't figure out a reason why the format is changed such that we are adding extra M's and d|D's to the format when v5 strictly followed the given format.

Your environment

npx @mui/envinfo

``` System: OS: Windows 11 10.0.22631 Binaries: Node: 20.16.0 - ~\AppData\Local\Temp\xfs-af08357a\node.CMD npm: 9.6.7 - C:\Program Files\nodejs\npm.CMD pnpm: Not Found yarn: 4.5.0 (this is what we use) Browsers: Chrome: 128.0.6613.139 (Official Build) (64-bit) Edge: Chromium (127.0.2651.74) ```

Search keywords: date picker format

mui / mui-x