Belatti, can I make some basic observations, please, about the plots you reproduced?
First it is not clear what "ride height" means. Is it front axle, rear axle, average? If average, is the rake held constant? If average and a constant rake, then the D/F would be expected to increase as the platform moves towards the ground plane, until the airflow starts to break down. I would not expect this to happen at 13 mm. (is that a model ride height, or a full scale ride height, BTW). Overall, the red plot has much less variation with ride height than I would expect.
When executing "first cut" aeroelastic work, it is common (in the aircraft world) to assume that aero forces are as steady state, but modified by a lag-lead filter, called the Wagner function. This is based on the assumption that airflow takes time to develop after a change in condition. The approximation implies that half the D/F develops immediately, whilst the remaining half builds up fairly slowly until the full static value has been reached. When the aero "platform" is oscillated it implies that the change in D/F should be rather less than the "steady state" change, and should be delayed in time when compared with platform position.
That is the assumption I made when I analyzed the motion of a skirted ground effect vehicle, & it worked reasonably well (I thought at the time).
I can't see anything like that characteristic in the blue plot. However, if I had generated the result, I would certainly be checking my "live mass" correction algorithm, because it does look as if the model inertia forces are included (wrong sign in the live mass correction would be my first port of call).
If the live mass corrections are correct, then what mechanism would increase peak D/F dynamically, but not statically, when the platform is close to the ground plane? If you replied "flow breakdown", then why is there no evidence of flow re-attachment in the steady state results at greater platform ride heights?
I remain puzzled.