The usual way how to deal with this problem is using query, not individual fields in the dialog. You’ll press Select button in the dialog and set filters as usual in SysQueryForm. This is supported by reports too - and widely used.
Multiple values from customized lookup in report will never work. instead you can take the datasource which contains item id field and assign it to your report. Then by default you get select button in report dialog and there you can specify multiple items then.
Removed datasources and redesigned the report using fetch method and the multple value filter issue got solved. No code for filter …rather it is working as in standard.