SPFx command bar buttons do not appear

Target SharePoint environment

SharePoint Online

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

💥 SharePoint Framework

Developer environment

None

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
• SPFx version
• Node.js version
• etc

Describe the bug / error

Starting on Sept 11th, some customers have reported that the command bar buttons deployed via an SPFx extension no longer appear. Nothing has been changed. So far, about 5 different tenants are affected. One of our own tenants is only affected when access from Ukraine.

We have not been able to identify any particular differences between the JS files loaded in the browser.

Steps to reproduce

As the problem seems to be affecting particular tenants only (with new customers appear to be affected each day), we can only assume it's due to some deployment being conducted by Microsoft.

Expected behavior

The buttons should appear as they always have.

No repository files or tests are named. Start by comparing affected and unaffected SharePoint tenants and the browser-loaded JavaScript, using the reported Edge, Chrome, and Firefox cases as the reproduction context. Done means identifying a reproducible cause and documenting a verified fix or mitigation for the missing SPFx command bar buttons.