Opening a Score Causes an Already Open Score to Get Closed

I've been working with Musescore pretty intensively for a couple of years now, and have yet to understand (or even predict very well) the rationale by which MS decides to close an already open score when I open some other score.

To produce the behavior I'm talking about:
1. Open any score.
2. Do as Save As to save it in the same folder as the original under a slightly modified name.
3. Re-open the original. Note that the copy gets closed as the original is re-opened.

Why does Musescore close the copy?!

I've never worked with another program that behaves this way. Opening one document never results in another open document getting closed, even if two of the documents are exact copies of each other in every particular except the file name. This permits one to make exploratory changes to the copy while keeping the original open for ready reference, comparison, and cannibalization.

Under most other circumstances, Musescore behaves the conventional way, which is to leave already open documents alone when a new document is opened. I never have quite understood the rules that produce the one behavior instead of the other; but then I don't understand why opening a file ever causes another file to get closed.

Can someone explain why Musescore does this?

Thanks!