dayjs().format('x') result output x characters

**Describe the bug**
Regarding the function of format('x'), the results obtained are inconsistent with expectations.
`dayjs('2023-09-01T08:12:16.468+00:00').format('x')`
output:x
`dayjs('2020-03-25T02:53:51.259Z')`
output:{$L: 'en', $d: Wed Mar 25 2020 10:53:51 GMT+0800 (中国标准时间), $x: {…}, $y: 2020, $M: 2, …}
`dayjs('2020-03-25T02:53:51.259Z').format('x')`
output:x
**Expected behavior**
Unix Timestamp in millisecond

**Information**
Day.js Version [1.11.9]
OS: [Mac OS 13.4]
Browser [Chrome Version 116.0.5845.110]
Time zone: [GMT +2]

The issue names no files or tests. First reproduce the two format('x') examples against Day.js 1.11.9, then trace the date-format token handling. Done means format('x') returns the Unix timestamp in milliseconds for both examples, with regression coverage added.

Written by the indexing model from the issue text.