When you combine frozen column with row selection and horizontal scrolling is applied (due to column and grid widths), column content are not hidden under frozen columns when a row is selected, as seen bellow in lines 2,3 and 4.
Checkbox, Product Name & Command columns are frozen. "Quantity per Unit" values are clearly visible behind frozen "Product Name" values. You can find a test scenario here (Telerik REPL for Blazor).
All themes suffer from this issue except Fluent theme. With default and bootstrap themes it happens on all odd selected lines and with material theme it happens on all selected lines.