Remote loopback hasn't be "broken for some years" but sometimes it does get, er, enhanced temporarily...but they do tend to fix it in the next f/w.

"The Pit" - how did you contact DrayTek/support - UK support do reply to all tickets

I hope you have sorted your issues out , I've ordered a Vigor2960

Had some replies back from Draytek.
Got confirmation that I've got the weighting right for load balance. However creating rules to send traffic down wan1 or wan2 still doesn't work. They're testing the loopback too see if it's working or not. However that was some days ago so I prompted them again today for more feedback.
When I get the traffic going down the designated wans.
I contacted draytek via the form on the uK website.

Draytek sent me a new beta firmware today.
The update didn't go smoothly as dhcp failed on restart. I then had to disabled and then re-enabled dhcp before the network would reconnect. I did think briefly that I may have bricked the router which didn't look good as router tools wouldn't connect to it so I upgraded via the web browser. I noticed a newer version of the software has been released so next time I'll try that first and if it doesn't talk to router doing beta testing may not be a good idea unless draytek promise to replace it.
Anyway end result same problems as before no local upnp and load balancing rules are ignored.

Is this a common issue at present, or just your unit ?
I've ordered a Vigor2960 to replace my 2830vn (plus) , original I went for a D-link DSR-1000n router with 2 new vigor120's as the d-link router has dual wan Ethernet but it had so many issues I've sent back and decided to stick with draytek

No idea it's only just come out. There maybe other issues with the firmware I don't know about as I don't use the feature.