I just checked Apple's documentation and found this note:

"If you upload a build and it remains in the Processing state for more than 24 hours, there may be an issue. To resolve the issue, submit a Feedback Assistant ticket or contact us."

Document URL: https://developer.apple.com/help/app-store-connect/manage-builds/view-builds-and-metadata#view-build-upload-status

I've already submitted a Feedback Assistant ticket myself.

However, since the US is celebrating Thanksgiving right now, I don't expect anyone to look at it until after the holiday.

I suspect this is a common issue that Apple needs to address for specific apps. So, if you run into this problem, don't just wait for it to resolve itself—be proactive and report it to Apple immediately.