Root Cause Analysis
Missing Viewport Meta Tag in Production: Based on the screenshots, the browser is rendering at a default desktop width (~980px) and scaling down. This only happens when the <meta name="viewport"> tag is missing or ignored. While client/head.html contains the tag, the Rspack Production Build may be bypassing standard Meteor head.html bundling, or the production URL is utilizing a different HTML template that lacks this critical tag.
URL of the Production website can be provided via PM if Meteor team wants to quickly test it.
@nachocodoner - this is another problem for your ever growing list 
UPDATE:
The Solution
I have moved the viewport definition directly into the Svelte root layout (App.svelte).
- Guaranteed Injection: By using
<svelte:head>, we force the browser to recognize the correct device width as soon as the JS app loads.