I've just answered your support ticket with this question and I am pasting my answer here for anyone else having a similar situation.
You can prevent items from being collapsed through the Overflow property: https://docs.telerik.com/devtools/aspnet-ajax/controls/toolbar/mobile-support/responsive-design#preventing-items-from-being-collapsed.
As for the items collapsing in a grid - the table layout of the grid must be set to fixed for this to work at all, because otherwise the table will take the width of its contents (the toolbar) and not the other way around:
I am attaching here an example based on the following resources that works fine for me:
If comparing it against your setup does not help you resolve the problem, please modify it to showcase the issue so I can investigate.