Duration is wrong on 1.11.10

**Describe the bug**
The duration is wrong.

Let's take this example

```javascript
// Init
const dayjs = require('dayjs')
const duration = require( 'dayjs/plugin/duration');

dayjs.extend(duration);

// two dates
const first = dayjs('2020-01-01T12:00:00.000Z')
const start = dayjs('2020-01-01T12:00:00.000Z').subtract(10, 'weeks')

// compute duration
const milliseconds = first.diff(start, 'milliseconds')
dayjs.duration(milliseconds).toISOString()
```

With dayjs 1.11.09, we get 'P2M10DT1H'
With dayjs 1.11.10 we get 'P2M9DT5H' which I don't even understand

**Expected behavior**

Previous behavior is right

**Information**
- Day.js Version 1.11.09 vs 1.11.10

Start with the supplied JavaScript reproduction using dayjs/plugin/duration and compare 1.11.09 with 1.11.10. Trace the duration conversion responsible for toISOString(); done means restoring the expected P2M10DT1H result without regressing the prior behavior.

Written by the indexing model from the issue text.