Is it possible to use getline in void functions?

Question

I have write a program and suddenly this is ignoring getline(cin, userInput); and executing all the functions in the assigned choice section. For example: If I want to encrypt the message, I will have to press E in the choice menu, and when I press E the program executes all the materials in the encryption function without asking me for the message that I want to be encrypted. But working fine with just cin >> userInput . What can be cause of this issue or maybe I am missing something to add.

#include <iostream>
#include <string>
#include <chrono>
#include <thread>
using namespace std;
void MainMenu();
void Decision();
void encryption ();
void decryption ();
void ExitProgram ();

int main()
{
   MainMenu();
   return 0;
}
void MainMenu()
{
   cout << "Enter any option that you want to perform" << '\n';
   cout << "=========================================" << '\n';
   cout << "E for encryption" << '\n';
   cout << "D for decryption" << '\n';
   cout << "Q to quite the program" << '\n';
   cout << "========================================" << '\n';
   cout << "Choice: ";
   Decision();
}
void Decision()
{
   char Choice{};
   cin >> Choice;
   switch (Choice)
   {
   case 'E': case 'e': encryption ();break;
   case 'D': case 'd': decryption (); break;
   case 'Q': case 'q': ExitProgram(); break;
   default:
      cout << "You have make a wrong decision! try again: ";
      cin >> Choice;
   }
}
void encryption ()
{
   string alphabets {"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"};
   string key {"XZNLWEBGJHQDYVTKFUOMPCIASRxznlwebgjhqdyvtkfuompciasr"};
   string userInput {};
   string encryptedMessage {};
   cout << "Enter the message you want to be Encrypted: ";
   getline(cin, userInput);
   cout << "Encrypting your message, please wait";
   chrono::duration<int, std::milli> timespan(1000);
   this_thread::sleep_for(timespan);
   cout << ".";
   this_thread::sleep_for(timespan);
   cout << ".";
   this_thread::sleep_for(timespan);
   cout << "." << '\n';
   this_thread::sleep_for(timespan);
   for (char c:userInput)
   {
      size_t pos_c = {alphabets.find(c)};
      if(pos_c != string::npos)
      {
         char pos_c_key = key.at(pos_c);
         encryptedMessage += pos_c_key;
      }else encryptedMessage += c;
   }
   cout << "The encrypted message is: " << encryptedMessage << '\n';
}
void decryption ()
{
   string alphabets {"XZNLWEBGJHQDYVTKFUOMPCIASRxznlwebgjhqdyvtkfuompciasr"};
   string key {"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"};
   string userInput {};
   string decryptedMessage {};
   cout << "Enter the message you want to be Decrypted: ";
   getline(cin, userInput);
   cout << "Decrypting your message, please wait";
   chrono::duration<int, std::milli> timespan(1000);
   this_thread::sleep_for(timespan);
   cout << ".";
   this_thread::sleep_for(timespan);
   cout << ".";
   this_thread::sleep_for(timespan);
   cout << "." << '\n';
   this_thread::sleep_for(timespan);
   for (char N:userInput)
   {
      size_t pos_N = {alphabets.find(N)};
      if(pos_N != string::npos)
      {
         char pos_N_key = key.at(pos_N);
         decryptedMessage += pos_N_key;
      }  else decryptedMessage += N;
   }
   cout << "The Decrypted message is: " << decryptedMessage << '\n';
}
void ExitProgram ()
{
   cout << "Thank you for using the program. Good By" << '\n';
   return;
}