Invalid format after timezone is changed

**Describe the bug**
```js
let dt = dayjs('2024-09-18').tz('UTC', true).startOf('day');

for(let i = 0; i < 4; i++) {
console.log(dt.format('YYYY-MM-DD'), dt.toDate());
dt = dt.add(1, 'day')
}
```
Result:
2024-09-18 2024-09-18T00:00:00.000Z
2024-09-18 2024-09-19T00:00:00.000Z << Wrong formatted date
2024-09-19 2024-09-20T00:00:00.000Z << Wrong formatted date
2024-09-20 2024-09-21T00:00:00.000Z << Wrong formatted date

My current time is 01:00:00! It seems format doesn't count timezone

**Information**
- Day.js Version 1.11.13
- OS: Ubuntu (docker)
- Browser: - (on a server)
- Time zone: [e.g. GMT+3:00]

Start by reproducing the timezone example with Day.js 1.11.13, focusing on tz(), startOf(), add(), format(), and toDate(). Trace how the timezone-adjusted value is updated across the loop; done when the formatted date remains consistent with the returned Date for each day, with a regression test covering the example.

Written by the indexing model from the issue text.