Hi @hikerdave. I don't have any idea what could cause this and I can't reproduce the fault on my own macOS machine. However, there was one previous report of the same bug:
Interestingly, they are also using macOS. That seems like maybe more than a coincidence because in my experience the great majority of forum users are on Windows.
If you are interested in doing a bit of investigation, I can provide some instructions you can follow to at least get an idea of whether the fault is in the Arduino IDE application itself, or in the helper tool that provides the communication between the Arduino board's serial port and the Serial Monitor GUI in the IDE. That would be quite interesting to know, but I can't promise that it would actually lead to a solution.
As a workaround for this bug, you can use an external serial terminal application. You would need to remember to disconnect that external terminal from the board before uploading, otherwise the terminal program's control over the port will cause the upload to fail. Arduino IDE is able to do that automatically with the integrated Serial Monitor.
I don't use macOS a lot so I don't have any suggestions for a specific terminal application, but I'm sure there are some good free open source applications available. The other helpers here on the forum might be able to make a suggestion.