Text Size Multiplying Automatically - Why?

I'm having an issue where my text size seems to be multiplying by 1000 for some DGN files (but not all) and i'm struggling to find any other setting to change or check.

I have checked the Working units in the Design File Settings, the Drawing Scale, Ensure annotation scale is off, the sizes in the Text Styles are ok, but it will still multiply by 1000, even when using a keyin.

For example if i run "active txheight 10" the text Size in the Dialog box will display 10,000

The only thing i can work out is that if my DGN file has a Design Model with working units of M / MM, the Sheet Model seems to multiply the size, even if the Sheet Model is set at MM / MM.

But if my DGN file has a Design Model with MM / MM, there is no problem with my Sheet Model..

Is there another setting i can check? 

Using MicroStation SS3