Kendo introduced a setting for each filter to be case sensitive or not, depending on a setting or derived by defaults (like case-insensitive for 'contains').

https://docs.telerik.com/kendo-ui/api/javascript/ui/grid/configuration/columns.filterable.ignorecase

However, when the Datasource for OData-v4 builds the filter, the setting is ignored and the filter always looks like:

$filter=Field eq 'Test'

or

$filter=contains(Field, 'Test')

A custom parameter map is needed to fix that behavior, however, it should be provided by the datasource directly. A case insensitive filter should look like this:

$filter=tolower(Field) eq 'test')

or

$filter=contains(tolower(Field)), 'test')

where 'test' has been transformed of the entered search value with: value.toLowerCase().