bug(cdk-virtual-scroll): Using cdkVirtualFor with ng-container doesn't render all items

#### Reproduction

https://stackblitz.com/edit/angular-afvrep

Steps to reproduce:
Click on the add button and new items will be added. Scroll down slowly until you see:
Item 49 didn't render well. If you scroll more down and back up item 49 will suddenly appear as needed.

#### Expected Behavior

All items should be rendered correctly

#### Actual Behavior

Some items (49 here) don't render as expected

#### Environment

-Angular: 10.2.2
-CDK/Material: 10.2.7
-Browser(s): Chrome Version 86.0.4240.111 (Official Build) (64-bit)
-Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

Start with the linked StackBlitz reproduction and reproduce the missing item while scrolling after adding items. Trace the cdkVirtualFor and ng-container virtual-scroll behavior; the issue is done when all added items render correctly during scrolling without needing to scroll away and back.

Written by the indexing model from the issue text.