When using Thunderbird with iCloud's `contacts.icloud.com` CardDAV server, any avatars that were made via Apple's ecosystem, do not have their avatars rendered correctly. Perhaps iCloud is using an image format, like HEIC that Thunderbird does not support.
I have noticed however, that Evolution Mail does support such contact avatars.
What the contact and its avatar looks like in the address book
What the avatar looks like when double clicking the avatar