As per Aero wash/loss of downforce when cars follow closely behind (1639513)
“Cars lose a significant amount of downforce when a car follows closely behind. This results in the leading car becoming extremely unstable and much slower through corners. This often leads to slow lap times and exponentially increased risk of crashing for the lead car.”
This issue has been partially addressed in update 3.0 FM Release Notes and Fixed Issues | Update 13.1 - 21 Oct 2024 - #6 by T10ManteoMax
It is still extremely prevalent and still ruins fair close racing.
A video with very clear proof: https://www.youtube.com/watch?v=kVbsO_NiFlM
Another test linked in the original thread: https://www.youtube.com/watch?v=IV9Wy7QMSfo&ab_channel=WiktorOlejniczak