Your latest sample prevents updating the other column when the value in the one you're editing doesn't match validation. This is not what I wanted. I will have to reassign values to the Combo to ensure the data that hasn't changed is still persisted.

I cannot tell what has changed to affect retaining the value.

Here is what I currently have for my column with a dropdown (not retaining value upon dropdown):

        <igx-column field="scenarioType" header="Type" [dataType]="'string'" [resizable]="true" width="83px" [sortable]="true">
            <ng-template igxCell let-cell="cell">{{cell.value}}</ng-template>
            <ng-template igxCellEditor let-cell="cell" let-value>
                <igx-combo [(ngModel)]="cell.editValue" (ngModelChange)="modelChange($event, cell)" [data]="types" width="220px" [igxFocus]="true" (onSelectionChange)="singleSelection($event)" [overlaySettings]="customOverlaySettings"></igx-combo>
            </ng-template>
        </igx-column>

A further frustration I've overcome is the use of @ViewChild with *ngIf in the grid. I figured out for myself that you need @ViewChildren when your component reference is gated by a conditional, which mine is due to asynchronously loading the list of Scenarios.

Here is what I have in the component.ts for this:

  @ViewChildren('scenarioList', { read: IgxGridComponent }) public scenarioList: IgxGridComponent;
  customOverlaySettings:OverlaySettings;
  types:string[] = [];
  ngOnInit(): void {
    this.getScenarios();
    this.getScenarioTypes(); //sets values to types:string[]
    this.getHorizons();
    this.getWorkweeks();
  }

ngAfterViewInit() {
    this.customOverlaySettings = { outlet: this.scenarioList.outlet };
  }

  modelChange(event:any, cell:IgxGridCellComponent) {
    if (this.inputSubscription) { this.inputSubscription.unsubscribe(); }
    let type:string = this.types.indexOf(event[0]) > -1 ? event[0] : cell.rowData.scenarioType, prefix:string = '';
    this.inputSubscription = of(event).pipe(delay(500), distinctUntilChanged())
      .subscribe(eventVal => {
        if (type == 'Commit' && !/^PLAN\_[0-9]{4}\_[0-9]{2}[a-zA-Z0-9\_\-]{0,38}$/.test(eventVal)) {
          this.badName = true;
          prefix = 'PLAN';
        }
        else if (type == 'Scenario' && !/^BETA\_[0-9]{4}\_[0-9]{2}[a-zA-Z0-9\_\-]{0,38}$/.test(eventVal)) {
          this.badName = true;
          prefix = 'BETA';
        }
        else if (type == 'LRP' && !/^LRP\_[0-9]{4}\_[0-9]{2}[a-zA-Z0-9\_\-]{0,39}$/.test(eventVal)) {
          this.badName = true;
          prefix = 'LRP';
        } else {
          this.badName = false;
        }
        this.badNameWarning = prefix !== '' ? `${type} Name should be: ${prefix}_YYYY_WW(optional [a-z,A-Z,0-9,-,_] to 50 total)` : '';
      });
  };

  singleSelection(event: IComboSelectionChangeEventArgs) { if (event.added.length) { event.newSelection = event.added; } };

I only filled out for the first 3 scenario types, but I assume it makes sense for the whole group. Validation is applied whether the Scenario Name or Type is changed. That portion is working like a charm and I am saving the Toast/ToolTip for save responses on the attributes of Scenario.

Regardless, the dropdown doesn't persist the populated scenario type when opened: no value is currently selected.