I am in contact with Seplos now. They sent me BMS firmware version 16.06.03. I tried it. No difference. Still only charged/discharged energy from the master, still not from the slave. All other reported values are correct, such as totals (e.g. power or current), averages (e.g. battery voltage) or min/max values (e.g. cell voltages).
I hope they take the problem seriously and come up with a solution. I suspect that the consumed energy in Victron is also wrong, caused by this same bug. VRM from ESS to offgrid ess keeps lingering in the dashboard - #7 by robmeerwijk