SplitViewController with 3 Columns - What am I missing??

Rick Aurbach

Xcode 13ß1, deploying to iOS 15, UIKit using Storyboards.
I have a tab bar controller which (for one of its tabs) contains a split view controller. The split view controller is configured in the storyboard as:
Style: Triple Column
Compact Wide: Use Separate View Controller - NO
Present Master With Gesture - YES
Display Mode: Two Columns Beside
Behavior: Tile
Primary Edge: Leading
I have triggered segues to the three (primary, secondary, and supplemental) view controllers set up in the storyboard.
The split view controller has a delegate which implements splitViewController(_: topColumnForCollapsingToProposedTopColumn:) to return .primary. Moreover, the split view controller's viewDidLoad() method reiterates the basic settings:
presentsWithGesture = true
preferredDisplayMode = .twoBesideSecondary
preferredSplitBehavior = .tile
With the Simulator set up to use an iPad in landscape mode, running the app shows ONLY the primary column. I do NOT get the "two columns beside" display mode.
Obviously I am missing something obvious, but I just can't see it. What am I missing and/or doing wrong?

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