Is it operator error for my cheek implant to be floating in my face?

Your doctor got one right so clearly he/she knows how to do a cheek implant. One ended up off the bone. Healing issues can occur. This sounds like a pretty easy fix. I would imagine this can be adjusted right in the office in about 20 minutes.

Implants are placed by creating a pocket and sliding the implant into that pocket over the desired area of augmentation.
Some surgeons place the implant into a snug pocket to avoid migration. If the pocket is too large, the implant can slide out of place. If it's too small, there can be too much pressure on the area, and the implant can extrude through the incision site.
To avoid this, many surgeons fixate the implant with 2-point fixation to avoid migration and rotation.