Wet umbrella? Dry it before folding: BLUNT's care guide
BLUNT recommends letting its umbrella dry fully, wiping marks with warm water and a clean cloth, and using the sleeve only after drying.
A wet canopy can stay damp if tightly rolled and placed in a sleeve. BLUNT's official care page recommends drying it open in a ventilated space, or upright with the handle up and fastening strap loosened where space is limited. Apply the instructions to BLUNT models; check other makers' guidance.