I don't use boots either. But it is easy to see the the issue. You have an additional leverage that you would not have with straps. That added with the concave gap has a twisting force sideways that the insert really isn't designed to take.
Exactly, thinking a second row of inserts allowing two points of contact either side of the boot would fix it although adding extra cost, work and weight.
As others have said. Put a shaped pad under the boot. That will stop the twisting force and kept it as an upward pulling force that the insert should be able to cope with.
If your insert has moving and loosened in the board then you will have to do a repair. If you have to go that then take the opportunity to add additional layers of glass around the boots for strength.
Yes for this particular board i just bodged some pads up but i was after a preventative fix for future boards as opposed to a reactive fix for this one. Downunders link above looks like a decent solution to try. Cheers.