STM32F108 as Slave won't work

This is an attempt to debug much bigger program:
I recently moved it to STM32, and ran into this issue:

The program never sees any packet addressed to it (address 0xB / d11)
I DO see and decode packets incoming on the I2C bus (from the master), using a scope connected to SCL1/PB6/22 and SDA1/PB7/23 .
I do NOT get to see them in the serial monitor.

#include <Arduino.h>
#include <Wire.h>
long lastms;
volatile int x = 0;
volatile int y = 0;

void receiveEvent(int howMany) {
  x = Wire.read();
}

void requestEvent() {
  y = 1;
}

void setup() {
  Serial.begin(115200);

  pinMode(PB6, INPUT);
  pinMode(PB7, INPUT);

  Wire.setSCL(PB6);
  Wire.setSDA(PB7);

  Wire.begin(0x0B);                // join i2c bus as slave
  Wire.onRequest(requestEvent);  // register event
  Wire.onReceive(receiveEvent);  // register event

  lastms = millis();
}

void loop() {
  if (millis() > lastms + 1000) {
    Serial.print("=");
    lastms = millis();
  }
  if (x != 0) {
    Serial.print("U:0x");
    Serial.println(x, HEX);
    x = 0;
  }
  if (y != 0) {
    Serial.println("V");
    y = 0;
  }
}

platformio.ini


[env] ; common settings
platform = ststm32
board = genericSTM32F103C8
framework = arduino
upload_protocol = stlink
build_flags = 
	-D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC
	-D USBD_VID=0x0483
	-D USBD_PID=0x5740
	-D USBD_USE_CDC
	-D USBCON
	-D USB_MANUFACTURER="Unknown"
    -D USB_PRODUCT=\"STM32F103C8\"
	-D HAL_PCD_MODULE_ENABLED
	-D HAL_I2C_MODULE_ENABLED
	-D ENABLE_HWSERIAL1
	;-Wl,--no-warn-rwx-segments
board_build.upload_method = stlink
monitor_port = /dev/ttyACM0
monitor_speed = 115200
lib_deps = 
	Wire


[env:for_stlink]
upload_protocol = stlink