Did a bit more troubleshooting yesterday. What I found was that in the phone app, the current always reads 0.0 A. The app is quite detailed and all voltages, per cell and the whole stack, seems to read out fine and I can see that it changes as I charge or drive.

Question remains why the current is 0 - it reads zero both when charging and when driving, so neither direction works as expected.
I checked the connections and found that I had made a mistake there, the terminals on the board had swapped the locations for "B" and "C" terminals compared to the original board - the written instructions were quite clear though (and labeling on silk screen) so just missed it when I installed it.

Even with my mistake I still think it would just read out as "wrong direction" but I suppose in worst case something broke from having it wired backwards. Still don't understand why both directions are bad though.

I did get a response from the YYBMS tech support through the Aliexpress message service (email addresses I found just bounced). They asked some questions related to if the app and display were showing the same thing and some other stuff. They concluded that the problem does not seem to be with the BMS but with the controller (not sure what controller they mean, but I assume they mean one on the bike or instrument cluster). That seems to be a dead end now, so I need to go after this myself to get to the bottom of it.

For now I think I will just leave it as it is since it works fine and I can keep track of charging by gut feeling, season lasts a few more weeks before it starts freezing in the nights and the bike gets parked. Then I might take another stab at this and/or possibly try to get a new BMS and see if problem stays or goes away.