UPDATE! Now I *am* embarassed!  I myself had posted a solution / workaround to this problem 3 Years ago! LOL!!   See the post, "Serial.read baud-rate issues..." in this forum.  In a nutshell, as Ben Rose pointed out, the processor just can't accurately do high baud rates (at least on Input).
So, the workaround is: USE 9600 Baud!   I just tested it, and it's working!
-------------
Hi y'all!  This question is embarassingly simple, but I'm stumped!  I'm talking to TinyDuino via USB Serial connection 115200 baud.  I send single-character "commands" from a Windows program to the tiny, pull them off the wire in the main loop() with:
  if (Serial.available() > 0)
      processIncomingByte(Serial.read());
and all is well.  Works great, no issues.   I've decided to expand my "protocol" a bit, to consist of multi-character "strings", e.g. - "DELAY 500", etc.  So I have tried this:
String str; // module declaration
in loop():
   if (Serial.available() > 0)
   {
     str = Serial.readStringUntil('\n');
     Serial.print("str = "); Serial.println(str);
     // also tried:  Serial.print("str = "); Serial.println(str.c_str());
   }
An example of the result (from the input, "M1234") is:
str = L⸮⸮
   I've also tried using char buffer[32], and appending single chars to that buffer, with a check for the '\n'.  Similar result.
What the heck am I missing here?  This ought to be falling-off-a-rock simple!
This doesn't work even with the Arduino serial monitor either; tho single-char inputs are fine, and all println outputs from the tiny.
  Any thoughts?

Hello Bullwinkle,

Could you attach your program's file? Should be faster to help you debug that way!

Did you try Serial.readString before trying the Until version? https://www.arduino.cc/reference/en/language/functions/communication/serial/readstring/

My last quick tip is to check that your baud rate in the code matches your baud rate in the serial monitor. Let me know if that helps!

Best,
Réna

Hi Rena,
   Please "refresh" this topic/post - I updated it 20 mins before you replied
readStringUntil('\n') works just fine.
Thanks for the reply!
Jim

Bullwinkle,

I am glad you were able to solve the problem! Sorry for missing the update!