Hi @semafox,
Indeed as we announced last month the new DataGrid will replace the legacy grid in the toolbox and newly created pages using Radzen templates however all existing pages will remain with the legacy grid. You can still however work normally with your existing pages with the legacy grid using Radzen property grid or you can even change the type of the legacy grid to the new data grid and vice versa in meta json:
In the next update later this week we will expose the legacy grid in the toolbox as Grid (legacy) and you will be able to drag & drop it to your pages. The border problem fixed immediately: