why did it use a static string to signal pty change?

src/main.cpp

@@ -205,9 +205,9 @@ byte tot;
 byte tunemode;
 byte unit;
 byte spispeed;
+byte programTypePrevious;
 char buff[16];
 char eonpicodeold[20][6];
-char programTypePrevious[18];
 const uint8_t* currentFont = nullptr;
 float vPerold;
 int ActiveColor;