Different behavior than standard Platform IO 'pico' when running simple Modbus RTU Server example using ArduinoModbus

[pomplesiegel]

Hello! I realize this is crossing streams slightly, but I'm wondering if what's going on here may be on a lower embedded level.

I am building a device using an Adafruit Feather RP2040 + a RS485 interface for Modbus RTU communication. Everything was working well when using the standard Raspberry Pi Pico platform on Platform IO. However, I really prefer the arduino-pico platform so I changed over. Everything was working great, but for some strange reason only 1 modbus server/slave running arduino-pico can be reached on a RS485 bus without waiting multiple seconds - a totally new and very strange issue.

I'm wondering if some strange timing / pin behavior is happening here under the hood?

This is the issue I created for the ArduinoModbus library, including a very short 20-ish-line generic MVP example of the issue.

Anyone have any ideas on what could be going on here? I've been beating my head against a wall trying experiments to uncover what could be the underlying issue.

Thank you!
Michael