Hi,
I have reproduced your issue.
After setting the integration of SharePoint, the default view of files sub-grid is empty and does not need to be modified manually.

And if you switched it to All SharePoint Document view in the new UI of Power Apps, the Document Associated Grid view will no longer be shown in the drop-down list. This part is actually not modifiable on the classic UI.

You can follow the next steps to set the Document Associated Grid as the default view.
- Create a new solution which contains the modified main form. Here, for example, I modified the main form of Account.

- Export it as an unmanaged solution.

- Update the customizations.xml from the downloaded Unmanaged solution zip file.

- Search for control id="DocumentsSubGrid" in the customizations.xml.
Replace the <ViewId> value A5B008AC-07D9-4554-8509-2C05767BFF51 with 0016F9F3-41CC-4276-9D11-04308D15858D.

- Import and publish the solution.

After setting these above, you’ll find what you need.
