Filters always affect sharpness. It's just a question of how much. It's one more set of air/glass interfaces. Basic optical science. It's one reason I only use the highest-quality multicoated filters. But in certain scenes, the apparent sharpness of a negative will actually improve simply because you might cut thru the UV or haze using an appropriate filter. Longer wavelengths are atmospherically scattered less than blue light, of course, so a red filter will often reveal details otherwise obscured by smoke or haze. Some films are susceptible to UV at higher altitudes, so a simply UV or skylight filter might help too. Then of course we use contrast filter to bring out certain things within a scene differentially. This
might make a neg more contrasty in the sense we desire it, and hence appear better differentiated in the print itself; but this should not be
confused with optical sharpness - nor should any of this.