Show Multicam Preview Monitor Bug

[Beta 27.1.0 b37] "Show Multi-Camera Preview Monitor" locks Program Monitor aspect ratio to 4:5 / square instead of 16:9

App Version: Premiere Pro (Beta) v27.1.0 (Build 37)

Previous Working Version: Premiere Pro (Beta) v27.0 (No issue)

OS: macOS

Issue Summary

In Premiere Pro (Beta) 27.1.0 build 37, enabling "Show Multi-Camera Preview Monitor" causes the multicam preview canvas inside the Program Monitor to restrict itself to an incorrect aspect ratio (approximately 4:5 / square).

The camera grid and live output render compressed into a centered box with massive empty grey margins on both sides, rather than conforming to the standard 16:9 sequence dimensions or filling the available panel space.

Steps to Reproduce

1. Open a multi-camera sequence in Premiere Pro Beta 27.1.0 build 37.

2. Ensure the Program Monitor is set to view Multi-Camera mode (Shift + 0).

3. Enable "Show Multi-Camera Preview Monitor".

4. Resize or dock/undock the Program Monitor panel.

Expected Result

The Program Monitor should scale responsively, displaying the camera matrix and program preview across the native 16:9 aspect ratio and filling the panel space (as it does in build 27.0).

Actual Result

The rendering viewport becomes constrained to a fixed ~4:5 vertical/square aspect ratio in the middle of the panel, flanked by large grey dead space.

Troubleshooting Steps Attempted

• Tested across multiple separate project files — issue persists globally.

• Reset workspace layout (Window > Workspaces > Reset to Saved Layout).

• Deleted the Layouts folder in Documents/Adobe/Premiere Pro (Beta)/....

• Reset preferences and cleared media cache (~/Library/Preferences/Adobe/Premiere Pro/).

• Switched Mercury Playback Engine between GPU (Metal) and Software Only.

• Cycled Playback Resolution (Full / 1/2 / 1/4) and toggled fullscreen (~).

• Confirmed regression: Downgrading back to version 27.0 completely resolved the issue with the exact same projects and media.