Loosing last byte with readBytesUntil

I have following code:
```
client_len = serverClients[i].readBytesUntil(_EOF, client_buf, MAX_FRAME_LENGTH);
client_buf[client_len++] = serverClients[i].read();
if (client_buf[client_len-1] != _EOF){
//something strange happened
serverClients[i].write("uhm");
continue;
}
```
How should I distinguish if _EOF character or MAX_FRAME_LENGTH or timeout has been reached?
The implementation reads the terminating character but then it is lost (not written to output buffer):
https://github.com/arduino/ArduinoCore-avr/blob/317513b9c989de24d438de7ac55d701b738881e2/cores/arduino/Stream.cpp#L224

See also: https://github.com/esp8266/Arduino/issues/6546

Start at cores/arduino/Stream.cpp around line 224 and inspect readBytesUntil's return value and terminator handling. Compare the behavior with the usage shown in the issue and the related ESP8266 issue. Done means the API or its documentation clearly distinguishes EOF, buffer length, and timeout, without unexpectedly losing the terminating byte.

Written by the indexing model from the issue text.