So, just to clarify things: -

Your Word 'Default paragraph style' is set to 'Normal' and this style is configured to use Calibri 12 point font. When you open a new document the style is set to 'Normal' and text is input as Calibri 12 point but when you insert a table, having checked first that you have not switched styles or changed the font by accident, the font in the table becomes Arial 12 point all by itself?

If this is correct then it may be a bug. I can't see how it is happening otherwise.