Thank you for reporting this issue. In the default code, the avatar background and letter colour are set together, and nothing in it removes only the background later, so something on your site or in the browser seems to be overriding it after the page loads. We need a little more detail to look into this.
Does this still happen on the current XenForo version with the default XenForo style selected?
Does it still happen in a private/incognito Chrome window with all browser extensions (e.g. dark mode or ad blockers) disabled?
Does it still happen with all add-ons disabled? (You can disable them temporarily by adding $config['enableAddOnHooks'] = false; to config.php.)
Did the new server or host add any front-end optimisation (e.g. Cloudflare Rocket Loader, LiteSpeed Cache, PageSpeed, or CSS/JS minification or lazy loading)?
When the colours are gone, which CSS rule does Chrome's 'Inspect element' show crossing out the avatar's background-color, and from which file?
This is an automated assessment. A member of staff may review and update it.