Why is the function of a projected resource reset, after the date is modified in services tab of the opportunity window?

In the opportunity (candidate) window, if the start or end date are modified after selecting a function for the projected resource, the function field is reset, i.e. either emptied or the default function value is selected automatically.
 

If a billing role or the start / end date are modified, the associated function is automatically populated. If the billing role has no associated function, then the function field is cleared.

Additional notes:
System Manager -> Financial Management -> Billing Offices (Cost Structures) page can be used for associating billing roles with a function, which is used as the default value in this context:
- select a billing office, go to Rates section,
- select "Staffing Level" in "View by"
- click the square shaped button "[...]" adjacent to the "Staffing Level" field
- select a function for each billing role