I got the same issue like you. For some reasons, at one time I used proxy (tried 2 3 proxies) WITH authentication, the code work correctly without issue.

Fast forward to recently, I used the same code but with a new proxy, it return empty html. Try tweaking it a bit but to no avail. However I found a quick remedy is that to use NON-AUTHENTICATED proxy, then it working perfectly. So you need to whitelist your IPs list which proxy allow, and use non-authenticated proxy instead.

Hope this help.