Thanks for the comments. We finally figured out how we could delete it after trying many things unsuccessfuly. We had to change the priority of the route, then we could delete. I have no idea why that's the case. Here's is the command we used:
conf iproute add xx.xx.xx.xx /29 xx.xx.xx.xx 200 <-- set the priority of the route low
After modifying it to priority 200, we could delete it ! Still seems like a bug.