This artifact is common after scanning for a long time with the same tip. Tips may last very long, or sometimes go blunt quickly. Some samples (known as "tip-eaters") have a tendency to lead to blunt tips very quickly.
In order to find out if your tip is blunt, scan a well-known sample like this one.

In the example above, the image on the right is made with a blunt tip, the left with a sharp (new) one.

How to avoid it

Use your tips carefully! If it gets blunt, replace it! In general, scanning with a lower applied force (see setpoint) helps to reduce tip wear.

Some low-wear tips which can scan for longer before becoming blunt are also available.