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The answer from Optimizely Support was that in an editor where there is no real focus you have to set it explicitly with onStartEdit and onStopEdit.
The updated code is:
_setValue: function (value) {
this.onStartEdit();
this._set('value', value);
this.onChange(value);
this._updateDisplayNode(value);
this.validate();
this.onStopEdit();
},
This solved the problem!