Check the style of text you are cutting and pasting. Type ST at the command line.
You'll probably find that the style you are copying exists in the new drawing and uses a differnt font.
If you type 'ST' at the command line and press enter, it will bring up the 'Text Style manager box. In the left window will be a list of the existing styles in your drawing. With each of these styles will be associated a font name, a height, width factor etc. If you copy text of this style (for example, style 'Romans') into a drawing which also contains the same style name ('Romans'),when you paste the text will inherit the style in the new drawing.
There may be a couple of other reasons this may be happening, but I would imagine this is the correct one..