[RESOLVED] Nano Every USB to Mac - Serial.print() mysteriously stopped working

Other Hardware 3rd Party Boards
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[RESOLVED]
There is a bug in the Print base class in package Arduino megaAVR Boards 1.8.5.
Downgrading to 1.8.4 avoids the problem. Serial.print(...) etc works properly again.
[/RESOLVED]

I have two Arduino Nano Every boards that I have been programming successfully from a Mac. This is using Arduino IDE after installing the USB serial port driver.

Both started having the strangest symptom at the same time. Calls to Serial.print used to work, but now arrive on the Mac as a smaller number of incorrect characters. Often a backward question mark, sometimes something different.

Programming them continues to work fine. Seems like a baud or parity problem at first but I don't think so. I've tried changing to 1200, 9600, 115200 and the boards can still accept correct characters via Serial.read() perfectly well when the baud rate is matching. Verified with tiny program that blinks onboard led based on what it received.

Has anyone seen something like that before? Could it be a serial USB driver problem on the Mac side?

Happened to two boards at the same time - and such a specific failure mode it's hard to imagine two boards had the same problem unless there's something I did to them. I'm trying to hook them up to RF24 modules right now.

Hmm... I wonder - I've never enabled "Serial1" UART but I did connect the RF24 IRQ output line to the TX1 output pin on the Nano Every instead of the RX0 pin. (TX1 and RX0 positions are reversed from what I expected based on pins D2-D12.)

Do you think that could have burnt out the "Serial" UART's ability to transmit data properly?

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Quick update:

I just tried to bypass the UART using a SoftwareSerial instance instead of Serial to communicate with serial USB.

#include <SoftwareSerial.h>
SoftwareSerial ss0(PIN_WIRE_HWSERIAL0_RX, PIN_WIRE_HWSERIAL0_TX);
#define Serial ss0

// and calls Serial.listen() after Serial.begin()

Exactly the same symptoms - ss0.read() works property and ss0.print() causes junk to arrive at the Mac.