Hi Isomorphic,

I think there is a problem with filterRow-SelectItem criteria. Depending on whether there is implicitCriteria it produces array-criteria or iContains-criteria.
I'm testing in 13.0p, because this version has the nice feature of a human readable hover description of the criteria, but it also applies to 12.0p and 12.1p.

I'd always expect the latter case for the SelectItem-criteria, as it is a selection and should not result in iContains, especially as the hover say this all the time:
"Field criteria: Government is one of commonwealth".
  1. Start sample
  2. Show filter row
  3. Filter for "commonwealth"
    Code:
    	Request:
    	data:{
    	        operator:"and",
    	        criteria:[
    	            {
    	                fieldName:"continent",
    	                operator:"inSet",
    	                value:[
    	                    "Africa",
    	                    "Asia",
    	                    "Australia/Oceania",
    	                    "Europe",
    	                    "North America",
    	                    "South America"
    	                ]
    	            },
    	            {
    	                fieldName:"government",
    	                operator:"iContains",
    	                value:"commonwealth"
    	            }
    	        ]
    	    },
  4. Click the null-implicitCriteria button
    Code:
    	data:{
    	        government:[
    	            "commonwealth"
    	        ]
    	    },

Best regards
Blama

Please see this modified testcase (v13.0p_2024-06-04):
Code:
isc.ListGrid.create({
    ID: "filterGrid",
    width: 850,
    height: 500,
    alternateRecordStyles: true,
    dataSource: worldDS,
    autoFetchData: true,
    groupByField: "continent",
    groupStartOpen: "all",
    sortField: "continent",
    groupByMaxRecords: 80,
    sortByGroupFirst: "true",
    implicitCriteria: {
        _constructor: "AdvancedCriteria",
        operator: "and",
        criteria: [{
            fieldName: "continent",
            operator: "inSet",
            value: ["Africa", "Asia", "Australia/Oceania", "Europe", "North America", "South America"]
        }]
    },
    fields: [{
        name: "countryCode",
        width: 60
    }, {
        name: "government", filterEditorProperties:{type:"SelectItem", sortField:"government", multiple:true},
    }, {
        name: "continent",
    }, {
        name: "capital",
    }, {
        name: "independence",
        width: 100
    }, {
        name: "population",
        width: 100
    }, {
        name: "gdp",
        width: 85
    }]
});

// Implicit criteria Buttons
isc.IButton.create({
    ID: "europeAsiaBtn",
    width: 250,

    title: "inSet: AF/AS/AU/EU/NA/SA (=all)",
    click: function() {
        filterGrid.setImplicitCriteria({
            _constructor: "AdvancedCriteria",
            operator: "and",
            criteria: [{
                fieldName: "continent",
                operator: "inSet",
                value: ["Africa", "Asia", "Australia/Oceania", "Europe", "North America", "South America"]
            }]
        });
    }
});
isc.IButton.create({
    ID: "europeAsiaAusBtn",
    width: 250,

    title: "inSet: AS/AU/EU/NA/SA",
    click: function() {
        filterGrid.setImplicitCriteria({
            _constructor: "AdvancedCriteria",
            operator: "and",
            criteria: [{
                fieldName: "continent",
                operator: "inSet",
                value: ["Asia", "Australia/Oceania", "Europe", "North America", "South America"]
            }]
        });
    }
});
isc.IButton.create({
    ID: "allCtnBtn",
    width: 250,

    title: "All continents (null criteria)",
    click: function() {
        filterGrid.setImplicitCriteria(null);
    }
});
isc.HStack.create({
    ID: "criteriaBtnStack",
    membersMargin: 30,
    height: 1,
    members: [
        isc.Label.create({
            contents: "Implicit Criteria:",
            height: 1
        }), europeAsiaBtn, europeAsiaAusBtn, allCtnBtn
    ]
});


// Filter criteria Buttons
isc.IButton.create({
    ID: "showFilterRowBtn",
    width: 250,
    title: "Show filterRow",
    click: function() {
        filterGrid.setShowFilterEditor(true);
    }
});
isc.IButton.create({
    ID: "hideFilterRowBtn",
    width: 250,
    title: "Hide filterRow",
    click: function() {
        filterGrid.setShowFilterEditor(false);
    }
});

isc.HStack.create({
    ID: "filterBtnStack",
    height: 1,
    membersMargin: 30,
    members: [
        isc.Label.create({
            contents: "Filter:",
            height: 1
        }),
        showFilterRowBtn, hideFilterRowBtn
    ]
});

isc.VStack.create({
    membersMargin: 30,
    members: [
        criteriaBtnStack, filterBtnStack, filterGrid
    ]
});