How to reproduce:

Create a new project and open it
Install the “Multiplayer Play Mode” package through Package Manager (“Window” > “Package Management” > “Package Manager”)
In the Unity editor next to the Play mode button, click on the “Default” dropdown and select “Configure Play Mode Scenarios”
In the opened “Play Mode Scenarios” window under the “Play Mode Scenarios” tab, click “+” button (“Create a new Play Mode Scenario”)
In the newly added “Play Mode Scenario”, expand “Additional Editor Instances” and click “+” button to create a new Editor Instance
Close “Play Mode Scenarios” window
In Unity Editor next to the Play mode button, click on the “Default” dropdown and select your newly added “Play Mode Scenario”
Open the "Active Scenario" window ("Window" > "Play Mode" > "Active Scenario") and ensure the "Keep Active" option of the "Player 2" instance is enabled
Enter Play mode and wait till 2 instances are loaded
Exit Play mode
Enter Play mode
Observe the Starting Scenario window

Actual result: The scenario stays in the "Deploying" state indefinitely and Play Mode is never entered
Expected result: The scenario finishes deploying and both the main Editor and "Player 2" enter Play Mode

Reproducible with: 6000.7.0a5, 6000.7.0a6
Not reproducible with: 6000.0.48f1, 6000.0.83f1, 6000.3.23f1, 6000.6.0f1, 6000.7.0a4
Could not test with: 6000.0.1f1 (Play Mode Scenarios do not exist in Multiplayer Play Mode 1.0.0)

Reproducible on: Windows 11 Enterprise (25H2)
Not reproducible on: No other environments tested

Workaround: Leave "Keep Active" disabled for additional Editor instances - the scenario then launches a fresh instance on each run

Notes:

In 1.4.1 version of Multiplayer Play Mode package to keep another player instance active, you need to select “Window” > “Multiplayer” > “Play Mode Status” > set “Manual mode” for the 2nd Player instance and click activate
To reproduce the issue "When entering Play Mode" ("Project Settings > "Editor") should be set to "Reload Domain only" or "Reload Domain and Scene"