Excel Add-in works on Mac & Web but not Windows

Question

Hello, all!

I've built a little Excel add-in that adds some custom functions to run calculations via a HTTP API.

I developed it on my Mac using the Yeoman generator, and selected custom functions with the shared runtime.

I modified the default functions, adding my own to call my API server to fetch results. Built and deployed to replit as a static site.

I tested it locally on my Mac via npm run start, which loaded Excel and ta-da! It worked! When I run the calculation, the cell shows BUSY while it talks to the API server (running on fly.io) then the number pops in.

Sideloaded the manifest.xml to Excel for the Web and ta-da! Works there, too. We're 2 for 2 so far.

Last test, Windows desktop. Created a network share and put my manifest.xml there. Added it as a Trusted Add-in Catalog in Excel for Windows. Then, the add-in shows and allows me to add it.

Taskpane loads. The "Run" button in the generated taskpane functions as expected, coloring the selected cells in yellow.

Typing a formula, my functions are there. All the names, autocomplete help, etc. are fine.

But, when I run the calculation on Windows, it says BUSY and thats it. If I recalculate, sometimes I get VALUE error, other times BUSY. Seems random. I get no errors, and dev tools in the taskpane shows no network requests.

I turned on dev logs in the registry, nothing useful there. No network errors or anything.

Looking at the API server logs, the Windows tests show zero activity. It's not even attempting to talk to the server. On Mac & the web, I see requests coming in as expected.

Any ideas? Why would it be working fine on my Mac & the Web but not Windows?