Maybe L/B still works after all 🙂
1. deleted both ppp0 and ppp1 interfaces from the load balancer.
2. disabled load balancer.
3. (probably not related) brought down both interfaces.
4. re-added ppp0 and ppp1 to the LB
5. re-enabled LB.
Now, after manually adding a dozen of segments to a couple of multisegment downloads, each of whom was going at 634 KB/s, I managed to push total speed to about 840-960 KB/s. This is reliable proof that load balancing is working, but unfortunately not as good as previously when it was always at 1.2 MB/s even with just 2 segments or 2 files going on. 🙁