At the same subject distance and the same true focal length, depth of field is mainly determined by aperture. For the common Canon kit lenses, the 18-55mm is typically f/5.6 at 55mm, while the 55-250mm can be f/4 at 55mm. That means the 55-250 should be capable of shallower depth of field, not deeper.

If what you’re judging is the DSLR viewfinder image, that can be misleading. DSLR focusing screens and viewfinder optics don’t show depth of field accurately beyond about the equivalent of roughly f/4 to f/5.6, because they are optimized for brightness rather than precise DOF rendering. So two lenses can look different through the finder even when the captured image won’t match that impression.

In short: the difference you’re seeing is most likely a viewfinder/focusing-screen effect, not because one lens is at the wide end and the other at the tele end. Compare actual photos shot from the same position, at the same focal length and aperture, to judge the real depth of field.