They did initially - the first two N gauge Mk.1 coaches they produced back in 1980 (SK & BCK) had raised frames round the windows (these were the original type with separate strips for the window area only rather than all-clear bodies). These two were the only ones they did like that though and subsequent models, starting with the RMB, only had raised marks for the door lines and handles, not the window frames. The construction was very similar to the Hornby 'shorty' Mk.3 that appeared a few years earlier ('77 I think). Again the original coaches had raised frames, but when they altered the standard trailers to an eight window pattern they didn't bother moulding frames (though they had already done so for their TRUB).
Certainly there were occasions when there were unprinted parts round the raised frames on the Farish Mk.1s, so that might be why they dropped them quite quickly.