Exporting COCO format only exports some tasks

**Describe the bug**
I've imported a large number of tasks with annotations using the JSON format to import.

I've then clicked through every annotation and checked it for quality.

When I export YOLO format it works fine, but exporting coco the results.json only includes 80 or so images and their annotations. I need Coco format for MMdetection training.

**To Reproduce**
Steps to reproduce the behavior:
1. Create project
2. import JSON with roughly 1200 images and annotations
3. spend the best part of a day clicking through each one and adjusting the annotations
4. click export
5. select COCO
6. check result.json
7. cry.

**Expected behavior**
A full dataset with all the annotated tasks included

**Screenshots**


**Environment (please complete the following information):**
- OS: unbuntu unders wsl
- Label Studio Version (latsest, installed to day with pip install)