SPFx app - debugging and serving is fine, but after deployment, [object] [object] errors appear only on one tenancy

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

💥 SharePoint Framework

Developer environment

Windows

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS)
• mobile (Android)
• not applicable
• other (enter in the "Additional environment details" area below)

Additional environment details

• browser version - Latest
• SPFx version 1.18.2, 1.20, 1.21.1
• Node.js version 18.20.8, 22.18.0

All three tenancies this has been tested on conform to deploying to a site-collection level app catalog on a fresh Communications site.

Describe the bug / error

We have built many, many SPFx apps. The particular app in question exhibiting errors has worked correctly on the client's tenancy since last year (and still continues to do so). However, the client recently found a small bug in our code - we fixed it up and deployed a version to one of our development tenancies for testing. It was then that I noticed the app was showing [object] [object] errors on deployment:

Come to think of it, this app could have been like this for a long time on our tenancy - even restoring previous versions through the app catalog showed the same errors. I have tried upgrading SPFx versions, downgrading/upgrading npm packages, etc... Nothing worked. I tried provisioning new sites, new app catalogs, ensuring extensions coming from the tenant were disabled, and even changing the solution and webpart IDs in the package solution and webpart manifest, respectively. I have just tried provisioning a brand new 1.21.1 SPFx app and copying across the code to see if previous SPFx upgrades might have been the issue - still the same problem.

I have even followed everything from here: https://learn.microsoft.com/en-sg/answers/questions/2184648/unable-to-load-spfx-webpart-after-deploy

The console displays the following errors where I can track it to a particular file referencing Google fonts included in the bundle of our internal framework which is used in like 30 other apps (which all work fine).

Well, this is what I found:
On the whim, I decided to try to deploy the same app package to another one of our development tenancies - and it worked without a hitch. I even deployed all extensions and all other sppkg files to the app catalog in the other tenancy to confirm.

Steps to reproduce

No idea, the fact that it works on two tenancies except one is strange.

1. Build spfx app
2. Deploy to site-collection app catalog
3. 😭

Expected behavior

That sppkg file, on deployment, works irrespective of tenancy.