When using the Material skin, RadFilter renders blank squares instead of action icons (e.g. add group, add expression, close, delete) when placed inside the ContentTemplate of a RadPanelBar.

This happens because .p-icon elements inherit font-family: "Material Icons" from the panelbar context, but the required icons are not part of that font. These particular icons are part of Telerik's internal icon font: "WebComponentsIcons".