When passing Data prop to Grouped mode, the outermost indexs can disappear [BUG]

[DeeBndx]

Describe the bug
When using Grouped mode, and setting the "data" prop the two outermost items (first and last index) will not be rendered depending on configuration of the "itemContent" prop.

Reproduction
I've made an example at: https://codesandbox.io/s/laughing-platform-hkcote

To Reproduce
Steps to reproduce the behavior:

FIRST ITEM MISSING:

1. Pass data to <GroupedVirtuoso data={listOfItems} />
2. In itemContent use the data parameter to render items from the list eg .
   itemContent={(index, groupIndex, itemData) => { return ( <div style={{ padding: "16px 0" }}>{itemData.content}</div> ); }}
3. The first item in the created list will not be shown.

LAST ITEM MISSING:

1. Pass data to <GroupedVirtuoso data={listOfItems} ... />
2. In itemContent use the index as selector for the list to loop eg.
   itemContent={(index) => { const currentItem = listOfItems[index]; return ( <div>{currentItem.content}</div> ); }}
3. The first item in the created list will not be shown.

Expected behavior
When passing the "data" prop to the component, I expect to see all items in the given data set.

Screenshots

Desktop:

• OS: Windows
• Browser: Brave, Chrome, Firefox, Edge

[petyosi]

Duplicate of #235