According to Advanced searching - functions reference:

For the startOfWeek() function the result depends upon your locale. For example, in Europe the first day
of the week is generally considered to be Monday, while in the USA it is considered to be Sunday.

But, seem not the case where only able to search start on Sunday issue.

Steps to reproduce the problem:

1. Create issues in a project on Sunday and on Monday
2. Change the time zone in the personal profile to Europe and GMT+1 or GMT+2.
3. Change the Default users' time zone in the Jira Settings > Systems > General Configuration.
4. In the Look and feel section, enable Use ISO8601 standard in Date Picker in the Date/time formats.
5. Look for issues created after startOfWeek().
6. The issues that were created on Sunday and Monday will be displayed and not just after Monday.

Please investigate on what factor determines the Locale of the instance, as modifying the time zones in the Personal Profile and the General configuration does not bring about any changes in the search results.