startOf('isoWeek') not as expected

code:
dayjs.extend(isoWeek);
const result=dayjs('2021-01').startOf('isoWeek');
result:
2020-12-28

I look up the information，ISO 8601:
ISOweek define: The week of the first Thursday of the year;

so dayjs('2021-01').startOf('isoWeek') is. 2021-01-04。

I read dayjs code ，does not filter thursday。

**return "isoweek" === i.p(e) ? s ? this.date(this.date() - (this.isoWeekday() - 1)).startOf("day") : this.date(this.date() - 1 - (this.isoWeekday() - 1) + 7).endOf("day") : n.bind(this)(e, t)**

I think like this：
**return "isoweek" === i.p(e) ? s ? (this.isoWeekday() < 5 ?
this.date(this.date() - (this.isoWeekday() - 1)).startOf("day")
:this.date(this.date() - 1 - (this.isoWeekday() - 1) + 8).endOf("day"))
: this.date(this.date() - 1 - (this.isoWeekday() - 1) + 7).endOf("day") : n.bind(this)(e, t)**

Reproduce the dayjs.extend(isoWeek) and startOf('isoWeek') example from the issue, then inspect the isoWeek plugin's startOf path and compare it with the ISO 8601 week definition. Confirm whether the observed date is incorrect and add or update regression coverage if needed.

Written by the indexing model from the issue text.