Multi-selection for label instances not working

**Describe the bug**
When labelling data with bounding boxes and choice labels (per region) it should be possible to label multiple objects at once. This is not possible at the moment - when I select multiple objects, the choice label disappears.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a project with labelling template:
```

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2. Draw a few objects/regions of any type.
3. Select more than one object at once,
4. The "color" choice tag is not visible anymore.

**Expected behavior**
The choice label shouldn't disappear. It should be possible to add a label to multiple objects/regions at once.

**Screenshots**
When choosing one object the color choice is visible:
When choosing multiple objects the color choice is not visible:

**Environment (please complete the following information):**
- OS: Ubuntu 20.04 LTS
- Label Studio Version 1.7.3

**Additional context**
This is not in accordance with the documentation - this functionality was introduced in version 1.3 - https://github.com/heartexlabs/label-studio/issues/655