Why Content Delivery in Sitecore 9.3 is treating users as editors?

This is the support help I got and fixed it: Could you please try overriding the CheckDomainGuard processor using the following:

namespace CustomNamespace
{
    public class CustomCheckDomainGuard : Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.SignedIn.CheckDomainGuard
    {
        public CustomCheckDomainGuard(BaseLog log) : base(log)
        {

        }

        public override void Process(SignedInArgs args)
        {
            if (string.Equals(args.Site.Name, Sitecore.Constants.ShellSiteName, StringComparison.Ordinal))
            {
                base.Process(args);
            }
        }
    }
}

Then please use a patch file containing the following to patch it in:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:set="http://www.sitecore.net/xmlconfig/set/">
    <sitecore>
        <pipelines>
            <owin.cookieAuthentication.signedIn>
                <processor type="Sitecore.Owin.Authentication.Pipelines.CookieAuthentication.SignedIn.CheckDomainGuard, Sitecore.Owin.Authentication" patch:instead="*[@type='CustomNamespace.CustomCheckDomainGuard, CustomAssembly']" resolve="true" />
            </owin.cookieAuthentication.signedIn>
        </pipelines>
    </sitecore>
</configuration>

Sitecore support has registered this a bug with reference number 366550, and there is a hotfix available for 9.2 and 9.3 here.