Confirmed. It was the motherboard not managing the displayport chipset correctly. Using the Thunderbolt 4 Add-In Card (and after enabling TB in the UEFI) I was able to use the KVM perfectly.
Thank you for your help ! 
Confirmed. It was the motherboard not managing the displayport chipset correctly. Using the Thunderbolt 4 Add-In Card (and after enabling TB in the UEFI) I was able to use the KVM perfectly.
Thank you for your help !