That is the Achilles heel of the bumper cover mount. As another poster illustrated that little flange you have remaining is supposed to extend further and have a hole in it. That hole snaps over a peg in a bit of plastic that is held in place by the lower headlight bolt. That bit of plastic is also prone to breakage, and to replace it you have to remove the bumper and the headlight. My guess is that if the bumper cover ripped there that plastic piece is also broken.
It might be possible to rig up a fix for the broken hole in the cover, but I strongly doubt that any form of epoxy, plastic weld, etc. will hold there.
As an aside, sometimes this doesn't work even if both the cover and the plastic piece are intact. The P5 my daughter now drives was in a minor accident, hit on the passenger side of the front bumper, cracked that headlight. The body shop that fixed it (under her direction, I was 1800 miles away and couldn't help) replaced the headlight and basically just stuck everything back together. The front bumper is mashed a bit under the cover - it is normally a rectangular tube and it is no longer square. This is just the last few inches that are messed up, the styrofoam energy absorber is intact except right there. When I finally got my hands on it I could see that the little plastic lifter adjustment that sits under the headlight had snapped off and not been replaced, and the plastic peg for that corner was also broken. So ordered those. When the bumper cover came off all the way only then did it become apparent that the bumper was not square, so those lifters wouldn't sit properly. This was kludged with a couple of washers, which put that back in more or less the right attitude. The plastic peg bit was replaced. Tried to snap the bumper cover back on and the peg and hole didn't line up - the peg is now about 2mm posterior to the hole. The bumper cover cannot be pushed back any further because it smacks into the fender. It seems that at least the lower mounting point for the headlight was pushed back a few mm in the crash. So now that flap sits on top of the peg, not around it. I have vague hopes that it might deform into position at some point, like on a nice hot day. Probably not. At some point I need to revisit this, maybe a washer between the plastic bit and the fender will move it forward enough. Also the headlight aim is probably off a bit now.
For what its worth, the other side has the hole over the peg, and has a much larger gap. It has been like that since I got it. The previous owner had had somebody back into the driver side front fender. They replaced that, but things are not quite square over there either. It looks like that fender is too long somehow, and that pulls the bumper cover down more than it should. The 2 bolts that mount the cover to the fender are in good shape on both sides, that seam is closed. I even replaced the grommets on the driver side, so that the bolts would get the best possible grip, and not "spin" like they did in the old ones, and it didn't make any difference. (These grommets are easy to replace, reach up with a pair of needlenose, grab them, and snap them off. Then snap in the new ones.)