Hello,
1) lot of such static pages are cached; thus loading it without Avast Web Shield running may lead to subsequent loads not really loading the page
2) Browser keeps connections alive; when enabling webshield, only subsequently created connections are scanned.
To test the issue, you would probably need to clear history and restart the browser.
We are not able to reproduce the problem on Catalina.
Kind regards,
Ondrej Kolacek