Yup, just happened to me in the exact same spot. Underground fort, to the left and down the slope, just under the ridge that goes overhead. Bulette seems to be stuck digging underground in one place, not moving. If my characters move over bulette's earth mound, they get knocked prone, but the combat doesn't start, and the bulette doesn't move away. The only way to get unstuck is to use fast travel, and bulette doesn't spawn for combat in the usual location as it's stuck here permanently.
