Pixhawk stuck in bootloader when connected to companion pc

I am experiencing a weird situation where Pixhawk (Pix32v5) is stuck in a bootloader when I have an unpowered companion PC connected to TELEM1. The companion PC is powered from its own power rail, so if I connect to Pixhawk using USB, it stays off. But this also means that Pixhawk never loads past the bootloader phase. If I physically disconnect the onboard PC, all is fine. Any ideas how to fix this?

The onboard PC is connected using UART RX/TX, CTS/RTS and GND.