I also activated the device’s overall QoS, setting a total device limit of 6146/768, and rebooted, and suddenly the per-session limits are followed for both up- and downlink. 256/128 nicely gives 260/112.
Not sure if it was the reboot that did it, or maybe the general activation of QoS service (maybe that service needs to be active for this?) but I am happy, and think I shall stop tinkering with it now.
Edit: Some further info: Setting a total upload QoS limit in DD-WRT seems to choke the router somehow. If I have any value in the upload, I can never get a total bandwidth higher than 400, but if I leave zero (no upload limit) in DD-WRT it goes to 800, but then the per-session limit is not enforced again.
Rather disappointing. Maybe its all firmware related, maybe its more generic.
If possible, please post below if you find you have a DD-WRT (with another build) where per-session limit is followed, but setting the QoS does not kill the total upload.
Thanks