Question:

Checkbox group has multiple selection every time. What may cause this behavior?

Answer:

The issue with multiple selected behavior on exact field may caused by wrong field mapping.
For example, the checkmarks regions in group are mapped not on the checkmarks exactly, but on the description text next to them, which leads to wrong recognition of the group contents.

Make sure that checkmarks areas are mapped on the checkmarks or empty checkboxes only, without any text or other irrelevant image objects inside the checkmark regions: