Thies Rubarth, I haven't been able reproduce this with a simple entity as follows:

@Entity
@Audited
public class TestEntity {
  @Id
  private Integer id;
  private BigDecimal value;
  @NotAudited
  private String text;
}

I've tested several scenarios

1. Update entity text property, yields no audit row.
2. Update entity text and value propery where value is set to a new instance of BigDecimal with the same double value, yields no audit row.
3. Update entity value property with a new instance of BigDecimal with different double value, yields audit row.

It seems regardless of scenario, the expected behavior is observed. Perhaps you could attach a test case that highlights the problem more clearly?