A sample for reproduction: https://codesandbox.io/s/overview-devextreme-toolbar-forked-eb4yd?file=/App.vue
As a temporary solution, define DxSelectBox within the default item template: https://codesandbox.io/s/overview-devextreme-toolbar-forked-e8b2d?file=/App.vue
We have addressed the issue described in this ticket and will include a fix in our next maintenance update. Should you need to apply our fix prior to official release, you can request a hotfix here.
Note: Hotfixes may be unavailable for betas/updates set for release within a week.
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