Reports function - possible bug

Reports function - possible bug

Hi,
Whenever I create a new report and save it, then want to make a change, and click 'Customise', any 'lookup fields' I had selected to display as columns in the report no longer display. At first I thought that it may be because of invalid characters in the field names (some of the fields have - in them. What characters are valid for field names by the way?), but then I realised only lookup fields seem to be affected. Report runs fine from save until I click 'customise', then the fields disappear, even if I don't actually make any changes. I'd like this fixed because I usually make one report as a template, the use the 'Save As New' button to save copies with filtering changes.
 
Thanks,
 
Jackson