How do I make a proper long string scanner to detect specific tags in a stream of characters? Here's my code:

#include &lt;SoftwareSerial.h&gt;
SoftwareSerial BTserial(2, 3); // RX, TX

void setup() {
    Serial.begin(9600);
    BTserial.begin(9600);
    Serial.println("Communication with HC-05 successfully started.");
}


bool is_uploading_book = false;
char c = ' ';
String recv_scanner = "";
void loop(){
    if (BTserial.available()) {
        c = BTserial.read();
        recv_scanner += c;
        if(recv_scanner.length() &gt;= 50) {
          recv_scanner.remove(0, 1);
        }
        // I believe there's something wrong with the recv_scanner.
        if(is_uploading_book) {
          Serial.write(c);
        } else {
          if(recv_scanner.indexOf("(__BOOK_UPLOAD_START__)") &gt;= 0) {
            is_uploading_book = true;
          }
        }
        Serial.println("\n\n" + recv_scanner + "\n\n");
    }
}


I'm sending a book (long text) in the form of characters one by one from my phone to my HC-05 module (and the Arduino).
My goal is to spot the tag "(_BOOK_UPLOAD_START_)" in the incoming characters.
Arduino allows me to receive the characters of the book one by one on each loop.
I'm storing those characters in a string called recv_scanner where I allow it's maximum size to be 50 characters and once it reaches those 50 characters, I start to remove the first character and add a new one on each loop (so it can only see 50 characters at a time because maximum String length is 200 and for my purposes, 50 is enough).
The thing is, it doesn't work properly.
When I Serial.println the recv_scanner on every loop (so I see how it progresses) it starts to look like this:

⸮qsnieq⸮cn .laslvanm.⸮⸮⸮d⸮alvuv⸮⸮urosqiam tr⸮⸮⸮


(It starts normally and then slowly progresses into this)
I have no idea what that is.
As far as my predictions go, it should show 50 characters and they should be parts of the books.
Is there something fundamental about Strings that I don't know of?
I'm doing this so I'm able to detect a tag (_BOOK_UPLOAD_END_) at the end of the book and finish uploading.
Feel free to ask my questions if something is unclear.
Another option is to not save the characters in buffer but instead check each one as they arrive to see if the character is the next in the tag sequence.

This has the advantage that you don't need a buffer to match the tag and you don't have to repeatedly search for the string in the buffer.

const char* TAG_STR="(__BOOK_UPLOAD_START__)";
int tag_pos = 0;
char c;
bool is_uploading_book = false;


void loop() {
  if (BTserial.available()) {
    c = BTserial.read();
    if (is_uploading_book) {
      //do book stuff
    } else {
      if (c == TAG_STR[tag_pos]) {
        // c matched the next character in the string
        tag_pos++;
      } else if (c == TAG_STR[0]) {
        // matched first char in the middle, restart
        tag_pos = 1;
      } else {
        // Did not match the next character so we should start over
        tag_pos = 0;
      }
      // The next character to match is the string terminator
      // so we must have matched the whole string.
      if (TAG_STR[tag_pos] == '\0') {
        is_uploading_book = true;
      }
    }
  }
}