Yes, the problem is with the new Serial Monitor in IDE 2.0 that needs Ctr+Enter to terminate the string. I totally agree with you that this is a most unfortunate change.
Running the same sketch built using the 2.0.0 beta 5 IDE, but using the Serial Monitor in IDE 1.8 shows that the embedded sketch's operation is 'normal', that is Serial.available() returns non-zero when the CR is entered. Using the ucon terminal emulator the behavior goes even further, responding immediately when each character is entered, not waiting for a CR.
I confirm that Serial.available() is working correctly.
Thanks for you help.