In the template editor in VisualEditor you can add parameters to a template via an input widget with a suggester dropdown. You can either pick parameters that are from a pre-defined list of parameters from the template's TemplateData documentation. Or you can add your own parameter if you want.
This suggester sometimes blocks you from adding a parameter back that was removed before.
The reason for this misbehavior is that the suggester holds a reference to the template model. The suggester is reused since https://gerrit.wikimedia.org/r/698547. But the template model is not always the same. It might be a new one. From this point on the suggester pulls data from an outdated model.
This causes a wide variety of confusing misbehavior.
One way to reproduce:
- Edit any article with VisualEditor.
- Add a template, e.g. "Infobox Film".
- Add an arbitrary parameter, e.g. "q".
- Don't enter a value.
- Edit the template again. The parameter is gone. That's fine. It was empty, that's why it was removed.
- Add "q" back.
- Save again.
- Edit again.
- Try to add "q" back again. From here on it doesn't work any more.