Calc: #ref error when referenced files are closed, no error when open

So I have this Calc file (target) with references to cells in other files (source).

When those source files are not open, and I open the target file, first there is this progress bar on the bottom showing that the source files are being read; after progress completion, the cells in the target file give me #REF error.

This error was not there with just 2 or 3 source files; it appeared when files became many.

To get rid of the error, I have to open all the source files in Calc, then reopen the target file.

In other words, for references to work, I have to keep all the files, both source and target, open.

Any ideas what’s happening? Thanks.