Why does Excel in Office 365 mess up a UNC format filename by dropping \\?

I'm pulling filename out of a database, and pasting them into Excel. Some of them have paths mapped to a drive. Those all work.

Others are UNC-style names, starting with something like \\util3\...

For those, everything works fine for me. I have MS Office Professional Plus 2013 installed.

Another user has Office 365, and for him, the entry in the cell is missing the \\ at the beginning of the path. So the filename isn't functioning like it should.

BTW, we're not creating hyperlinks. The filename is inserting into a text box on a form, and should open when it's double-clicked.

We've both researched this but aren't finding a solution. Is it a glitch in Office 365? A setting? How do we get around this? It's a simple little thing, but we've spend several hours trying to resolve it.

2020-01-09T23:26:30Z

To be clear, this is NOT an issue with Excel converting to a mapped drive. It's showing the full UNC path, correct except for the missing \\ at the start.

2017-04-11T00:37:33.809Z