It is likely that the fusion protein is degraded, leaving a stable MBP-sized breakdown product. In this case, try using a protease deficient host. NEB Express is Lon- and OmpT-. A list of additional strains, which are free with an order or for the price of shipping, can be found on page 43. For cytoplasmic expression, the most protease deficient strain is CAG629 (NEB #E4125S) – it is also, however, the most difficult to work with. CAG597 (NEB #E4123S) is another good alternative. For periplasmic expression, the most protease deficient strain is CAG597 (NEB #E4123S); KS1000 (NEB #E4128S) and UT5600 (NEB #E4129S) might be worth trying as well. The CAG strains are difficult to transform, and often require electroporation to introduce the fusion plasmid.