I had those 2 scenarios today:
We killed the enemy bomb carrier and guarded the bomb. Still, the enemy team could plant. Despite the fact, that the bomb was still on the ground for us. All my team saw it, not just me.
Our bomb carrier was killed. I revived him, but he didnt seem to pick up the bomb again. So I took it and started to plant. Then the former bomb carrier came along and started planting too.
In the second case the carrier was a Fletcher. In the first case I am pretty sure it was a Fletcher as well. Both instances happend on Market Place.