Hi Wills,
Greetings from Syncfusion support
We have checked your shared information and we could see that when you set the frozenColumns as 8, the total frozen columns width was exceeding the Grid width. It was the cause of the problem.
By default, while using frozenColumns we must maintain the total frozen columns width is less than the Grid width, so only we need to display the movable content width horizontal scrollbar. It was the default behavior. Please refer the below modified sample for more information.
Regards,
Rajapandi R