Opening a second score opens a new window with a separate copy of MuseScore4. Visually, the first score just dissappears behind the second window. In the event of shutdown, failure, etc. the window may be forced closed.
S3 - Major
The fist window is obscured by the second window. In the event of system closedown, the first window may or may not be saved before being closed. this leaves that score in an indeterminate state. This is a serious regression from MuseScore3.
Until full side-by-side documents are supported, it would be better if the opening of a second file forced the closure of the first file. Presently, the system is seriously erroneous-prone.
Reported countless times and currently by design