Just wanted to add to the topic some information to think about
https://www.researchgate.net/publication/343391137_CFD_Analysis_of_Slipstreaming_and_Side_Drafting_Techniques_Concerning_Aerodynamic_Drag_in_NASCAR_Racing
If we look at this graph
we’ll see that the leading car lose some drag when the car behind is approaching.
Not 100% about downforce, but in my opinion if it loses some drag then most likely it happens at cost of downforce, as the front of the leading car cant be affected.
Would be cool to see if someone tested side drafting too. If the effect you are talking about drastically increases during side draft, then it’s all makes perfect sense.