SSL Serverside Not Being Disabled Correctly

You should absolutely be able to use SSL::disable serverside in the HTTP_REQUEST event. You may alternatively want to adjust how you're filtering on the URIs, and/or investigate if these URIs are consistent.

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    HTTP::header insert "X-Forwarded-Proto" "https"

    if { [string tolower [HTTP::uri]] starts_with "/foo" } {
        pool pool_foo_https
    } elseif { [string tolower [HTTP::uri]] starts_with "/bar" } {
        pool pool_bar_http
        SSL::disable serverside
    } else {
        pool $default_pool
        SSL::disable serverside
    }
}