Yes—umbrella spill and room bounce can easily fill in shadows. A few low-cost ways to control it:

• Move the umbrella closer to the subject. Because light falls off quickly with distance, this makes the subject brighter relative to the walls, so reflected fill has less effect.
• Use black material to absorb stray light. Black velvet, black fabric, landscaping fabric, or other dark, non-reflective material on nearby walls can help.
• Add black flags or “bookends.” Large foam boards taped together can be very useful: leave one side white for reflection and paint the other side black to subtract light. Place the black side where you want to block bounce.
• Put a blocker near the light. A standing piece of cardboard, bifold doors, or simple DIY barn-door style blockers can stop spill before it reaches the walls.
• If possible, shoot in a larger space or outdoors at night, where there are fewer nearby surfaces to reflect light back.

In short: get the umbrella in close, and use black surfaces/flags to absorb or block spill.