Consider the following:

new OO.ui.MenuTagMultiselectWidget( {
	allowArbitrary: true,
	options: [
		{
			data: 'foo',
			label: 'Label for foo',
		},
		{
			data: 'bar',
			label: 'Label for bar'
		}
	]
} )

1. Click in the textinput
2. The menu is shown
3. type 'a'
4. The menu is hidden because no items match 'a'
5. type <enter>
6. Expected: A tag 'a' is added (because allowArbitrary)
7. Actual: A tag 'Label for foo' is added

Note: using menu: { highlightOnFilter: true } in the config fixes it but causes T209716

Note 2: Happens as described above only the first time after creating the control.