RPISER doesn't work on GrovePi+

I ran into some issue with RPISER port on GrovePi+ plate.

If I connect an external serial device directly to the RaspberryPi 3B+ serial pins (8,10), it works.
If I try to do the same thing trying to connect it via GrovePi+ RPISER port, then there’s no connection (and there’s no data on rpi3b+ serial port).

I.e. it works directly, but it doesn’t work via GrovePi+ plate.
Have I missed something obvious with GrovePi? I’d appreciate any ideas.

p.s. bluetooth is disabled in config.txt on raspberry, and there’s the working /tty/AMA0 serial port.