Interesting observation!
Questions for Roberto.

Since the IV has a blended wing, the tip section is only the tip section at the tip. As soon as you get away from the tip the section changes, and one would think the aerodynamic properties. The wing tip does not perform per the plots due to end effects (vortex generation etc.). Do you have any data or information about mid-span properties?

The non linearity you point out seems to only happen at  a Reynolds number of  3 E6. Does this correlate to the applicable flight  parameters?

Is this lift coefficient hysteresis  characteristic of some, many, all laminar sections?

As the section "thins" in the family 64-X12 the effect you point out becomes more pronounced. Do you think that manufacturing variances could reduce or exacerbate the effect?

What effect would the various winglet designs have?

Inquiring minds want to know.

Regards
Brent Regan