These labels still exist in annotations

I have a simple task containing a multiple-choice question, and once a choice is made, a second question would appear asking for sub-categorization. The data field really just has a 'text'.

Below is the code for the labeling interface:
```

```

However, once I complete an annotation and come back to the labeling interface, it always shows validation error:

> Validation error 5f48689b-e04c-4087-8910-c524ac366af0
> These labels still exist in annotations:
> ['A'] (1 annotations).Please add labels to tag with name="level-1".

Such a baffling error since everything looks valid. I have read the comments in[ this thread](https://github.com/HumanSignal/label-studio/issues/734) but didn't get any solution. Am I missing anything obvious here? I am running the latest version of label-studio on Windows 10.

Ps. The exported output looks ok, and if I remove the completed annotation and come back to Settings-labeling interface, the UI Preview shows up again.