Garbled serial output from ATtiny1616 when using Platformio IDE, OK on Arduino IDE

Code in PlatformIO IDE:

#include <Arduino.h>
void setup() {
  Serial.swap(1);       // Switch to alternate serial port.
  Serial.begin(115200);
  while (!Serial);
}
void loop() {
  Serial.println("Hello world!");
  delay(1000);
}

platformio.ini:

[env:ATtiny1616]
platform = atmelmegaavr
board = ATtiny1616
framework = arduino
upload_port = COM24
monitor_port = COM10
monitor_speed = 115200

When I build and upload this from PlatformIO IDE the serial output on alternate USART pins (PA1, PA2) is garbled (output looks like wrong baud rate):
If I use primary USART (delete Serial.swap(1)or replace it with Serial.swap(0)) the same output continues on alternate USART, nothing on primary USART. I tried Serial.pins() instead of Serial.swap() -> same result.
Same code built on Arduino IDE (except first #include line) works as expected, both on primary and alternate USART.

In both cases I program via UPDI / jtag2updi.

Any advise on what is going on here would be highly appreciated, as I prefer working in PlatformIO IDE.

platformio / platform-atmelmegaavr

Garbled serial output from ATtiny1616 when using Platformio IDE, OK on Arduino IDE #22