Thanks for your help Al.
I've created a new form with one textbox. It has the same behaviour. I'm starting to wonder if it has something to do with the font having unsupported functionality in the report builder !

The report environment is switched off both globally and on the report.

I've open the report table, the field "fontFace" correctly shows the font, but I need to check the value for field "fontStyle", it is "0" at present, fontSize = "0" and whether field "mode", "ruler" and "rulerLines" are used by the fonts.

Thanks again for your help.