USB Serial missing on Maple Mini

[canyouhearmenow]

I am trying to use a Maple Mini clone with MySensors, with an NRF24 radio wired to SPI1 as described in MyHwSTM32F1.cpp

• I can flash the board with a simple test script that has Serial.begin() in setup(), and it will present as a USB serial device to the Windows host (as COM23) ... so far, so good
• When I flash a script using MySensors, no USB serial device is presented
• When I redirect MySensors output to the "real" serial port with #define MY_SERIALDEVICE Serial1 and #define MY_DEBUG, I get the MySensors splash screen and all the expected log messages at the real serial output, but the script hangs after the call to Serial.begin() in setup()

Oh, and I'm doing all this in Platformio, and my platformio.ini both for the "working" simple script and the "non-working" MySensors script contains

platform = ststm32
framework = arduino
board = maple_mini_b20
board_build.core = maple
build_flags = 
    -D USBD_USE_CDC
    -D USB_MANUFACTURER="Leaflabs"
    -D USB_PRODUCT="\"Maple Mini\""
    -D ENABLE_USB_SERIAL
    -D ARDUINO_ARCH_STM32F1
monitor_port = COM23

How does MySensors interfere with the creation of the USB serial device?