Large program fails; shows strange LED blink pattern

MicroBlocks is a virtual machine (like MicroPython) that runs on a wide range of 32-bit microcontrollers. We use PlatformIO to build for all of them. MicroBlocks is a fairly large program.

I am porting MicroBlocks to the Raspberry Pi RP2040 chip, the Pico board in particular. It compiles and runs using Arduino IDE 1.8.13 (Mac OS) and Arduino's "Mbed OS RP2040" 2.0.0 board package this community supported package.

The same code compiles and installs with PlatformIO, but the program does not run on the board. The board does not appear as serial port (which it does when the program runs correctly), and the LED blinks in a strange pattern: four long blinks followed by three quick ones. The blink pattern could be an error code of some sort but I haven't been able to find any documentation that will help me decode it.

Here's the platformio.ini file entry I'm using:

[env:pico]
platform = https://github.com/platformio/platform-raspberrypi.git
framework = arduino
board = pico
lib_ignore = WiFi
build_flags = -D ARDUINO_RASPBERRY_PI_PICO

Start with the platformio.ini [env:pico] configuration and compare the PlatformIO build with the Arduino IDE build described in the issue. Investigate why the installed program does not expose a serial port and produces four long followed by three quick LED blinks. Done means the Pico runs the program and appears as a serial port when built with PlatformIO.