This is a problem for mw.widgets.TitlesMultiselectWidget, which inherits from OO.ui.TagMultiselectWidget and mixes in mw.widgets.TitleWidget.
Ideally the solution would be to rename the config in mw.widgets.TitleWidget, since it is the mixin, but that would have wide repercussions.
Would be less disruptive to rename the new config in OO.ui.TagMultiselectWidget.