Getting package is invalid `crx_required_proof_missing` when installing my own, signed extension

Question

I'm building an internal extension for my company. Since it's an internal tool, I don't want to publish it on the Chrome Web Store.

To be able to self-host the browser extension, I'm doing the following steps:

(before): I created a .pem to sign the extension (+ keep a constant extension id across updates)

1. I'm building the source code

2. I'm packing and signing the extension with chrome using chromium --pack-extension=./build_dir/ --pack-extension-key=./key.pem

3. I'm updating the update.xml file to reflect the new version number

4. I uploaded the signed .crx and update.xml

I have a website with a link pointing to the .crx file. The issue is the following: when clicking on it, the file is downloaded and chrome understands it's a browser extension. Then: I'm getting "Package is invalid crx_required_proof_missing".

What am I missing?

What can I do to be able to install a self-hosted extension after downloading the extension?

Notes:

I am aware that I can extract the .crx content and load the extension as "unpacked", but that's not what I'm looking for: deployment will be more complex (more manual steps) and updates will not be automatic.

The .crx file seems to be served with the right headers and is publically accessible. The update file is also correct and accessible.

Info:

Yes, the manifest contains properties key and update_url.

When dragging/dropping the .crx file in chrome://extensions (in dev mode): the extension is installed but I'm getting a red warning on the extension card: "This extension is not listed in the Chrome Web Store and may have been added without your knowledge"

When dragging/dropping the .crx file in chrome://extensions (without dev mode):

The extension is not installed and I'm getting: "Package is invalid crx_required_proof_missing".

When dragging/dropping the .crx file in chrome://extensions (with CHROMIUM and dev mode):

The extension is properly installed and working.