Reference card says that View Window (gray area in upper right corner of the screen), normally showing Might and Magic logo, turns into small overhead map if someone in party has the Cartography skill or Wizard Eye spell is active.

In fact, it shows a map only if Wizard Eye is active. Cartography only allows to use automap (M button) while View Window remains as gray and solid as headstone.

Is it a bug or a description error?

It's a description error, generalized from the newer games of the trilogy. It applies to World of Xeen (MM4+5) but the older MM3 only shows the Wizard Eye.

Thanks for a clarification. It's a pity it's not there, without neither Direction Sense skill nor Wizard Eye spell the exploration can be quite frustrating in M&M3.