When the field is at the top level if could be a priority issue. Try setting the priority of your filter to 20.
As for when it is in a repeater field, the reason for this is that the field name is not what you would expect. The field name in this case is constructed using this formula
"{$repeater_name}_{$row_index}_{$sib_field_name}"
More than likely the filter is not being called at all. For the case of this filter and repeater sub fields you need to use the field key rather than the field name. You will need to use the 3rd parameter that is passed to your filter to get other information when needed about the field.
