Keeping editor unfocused while in PIE leads to memory leak

Keeping editor unfocused while in PIE leads to memory leak, eventually freezing the editor. The editor must be closed through task manager and leaves no crash report or log error.

1. Download, unzip, open attached project

[Link Removed]

1. Let shaders compile
2. Press PIE
3. Alt+Tab to focus another window/screen
4. Open task manager and focus UE4Editor.exe in processes window, note memory usage
5. Allow editor to remain open in background during PIE for an extended period (user reports 30 minutes. Internal test was ~4 hours)

Results

Memory slowly builds and eventually spikes heavily, freezing the editor, forcing it to be closed manually.

Expected

Editor remains in background and does not require additional resources.