I don't have those unit / module so my conclusion could be wrong.
Looking at the Arduino code for the weigh module it seems that the clock timing / reading is quite time critical and if not perfect, the readings will be incorrect.
When connected to the PbHub the built in chip (MEGA328) would have to mimic the timing, but from the description I understand it can only handle simple GPIO, DA and AD. I don't think it can generate the necessary clock timing in the uSec range required.
Also the PbHub description mentions "Notice2: Not all M5 Units with PortB (Black) are able to be extended by PbHUB." Unfortunately it doesn't say which.
When you look into UIFlow, add a PbHUB and then select the weigh unit you'll notice that there are a couple of pin pairs to choose from, but PbHUB is not an option.
@M5Stack engineers: care to weigh in?