I make extensive use of the Serial.print statement to display debugging diagnostics. Lots of these messages happen during Setup and more occur during the Loop. There is a large amount of code involved, but when I add code, no serial output happens. If I add code in the loop, the code in the Setup section doesn't get displayed. I have the output logs enabled, and when this happens, the entire content of the log file is:

2018-11-29 16:21:25      Message      Opening port
Port open

I can make the output reappear simply by not executing some code late in the setup process. I put a "return;" statement in some long initialization code, long after lots of diagnostics have already been displayed. If I remove the "return;" statement, NO OUTPUT appears. How can this be? Doesn't make any sense to me.

The compiler output ends with:
Program size: 33,744 bytes (used 13% of a 253,952 byte maximum) (2.69 secs)
Minimum Memory Usage: 3980 bytes (49% of a 8192 byte maximum)

I am running this code on a Mega.

Same problem in both modes.

Thanks for the update. Yes class init is prone to happen before the arduino core loads. Surprised the cpu doesn't just crash.