Try disconnecting your stylus if you are using one. See if it acts the same when you attempt to smudge with your finger. If this stops it then there may be an issue with your stylus not working properly. If the problem continues, perhaps reset the brush you are using to its default settings.

Why is my brush opaque in procreate?

It’s that the default settings have an issue with the Opacity Limits Min and Max sliders, which are actually hidden below the visible panel in the General tab. Open the General tab and swipe up on the panel so you can see the Opacity limits, and set the Min slider to zero instead of 98.2%.

Why is my brush on procreate not working?

Try unpairing your Pencil (from Settings app > Bluetooth) and re-pairing it, and make sure the Pencil tip is tightly screwed on. These tips can sometimes come loose and cause issues like this. The pencil works for everything else.