The LoPy4 Pin-Out does not show other connections, but these may exist on an expansion board, like the VBatt divider at P16, the User Button at P14 and the Accelerator IRQ at P13.
@robert-hh thanks for this reminder; it solved my troubles eventually. One of the LoPy seems to be defective, but on all others, everything is fine now after moving around some pins. Thanks!