Different output dates when using the non strict parsing mode

**Describe the bug**
When creating a new date using the **non strict parsing mode** and including the **customParseFormat** plugin, I get the following results:
```
dayjs('201911', 'YYYY-M', false); // Jan 1st 2019
dayjs('201911', 'YYYYM', false); // Nov 1st 2019
dayjs('2019-11', 'YYYY-M', false); // Nov 1st 2019
dayjs('2019-11', 'YYYYM', false); // Nov 1st 2019
```
The first case returns Jan 1st 2019 instead of Nov 1st 2019
When removing the **customParseFormat** plugin, the output is correct: **Nov 1st 2019**

**Expected behavior**
All of these cases return **Nov 1st 2019**, according to the list of formats provided (https://day.js.org/docs/en/parse/string-format). The expected result should be the following when using the customParseFormat plugin:
```
dayjs('201911', 'YYYY-M', false); // Nov 1st 2019
dayjs('201911', 'YYYYM', false); // Nov 1st 2019
dayjs('2019-11', 'YYYY-M', false); // Nov 1st 2019
dayjs('2019-11', 'YYYYM', false); // Nov 1st 2019
```

**Information**
- Day.js Version: 1.10.4
- OS: Windows 10

Start by reproducing the four parsing examples with Day.js 1.10.4 and the customParseFormat plugin, then inspect that plugin's parsing path for non-strict formats. Done means all four examples return November 1st, 2019, including the unseparated YYYY-M input.

Written by the indexing model from the issue text.