Strange behavior with checkbox in Fragment

I refactored my code, moving from an Activity (containing checkboxes) to Fragment. It used to work as demanded (two-state checkbox) in the Activity but now for some reason the checkbox is a tri-state checkbox.

Fragment:

This doesn't work (tri-state):

chkMonday.setChecked(settings.getBoolean(getString(R.string.chkSettingsMondayKey), true));

The checkbox has a tri-state and I can't figure out why:

• default state when Fragment is loaded: empty with highlighted borders.
• one click: unchecked (gray)
• one click: checked (check mark + highlighted border)

Discovered that this works (two-state):

boolean monday = settings.getBoolean(getString(R.string.chkSettingsMondayKey), true);
chkMonday.setChecked(monday);

Do you have any explanation? Thank you!