In Av mode, this is normal behavior. The camera usually treats the flash as fill light for the subject, while still exposing the background with ambient light. That means it may choose a slower shutter speed so the background doesn’t go dark.

A few things are happening:

• Shutter speed mainly affects ambient light.
• Aperture affects flash exposure more directly.
• With flash enabled, the camera may also choose a lower Auto ISO, which can force a slower shutter speed.
• The shutter speed must also stay at or below the camera’s flash sync speed.

So the flash is not necessarily replacing the exposure; it’s often just adding light to the foreground while the camera still tries to record the scene behind it.

If you want a faster shutter speed, use Manual exposure mode and set the shutter to something like 1/200–1/250 s (near sync speed), then choose the aperture you want and let the flash light the subject. This gives you less ambient background exposure, but it avoids the long handheld shutter speeds you’re seeing in Av mode.