Unpredictable behavior while saving images into storage

Question

TLDR: Why isn't my app asking for runtime permissions despite I have written code for it?

I am saving an image into the storage, on button click by doing something like this:

// Function triggered when save button is pressed
private void saveImage(){
    // Asking Runtime permissions for API level 23 and higher
    if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.M){
        if(checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED){
            finallySave();
        } else {
            requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},STORAGE_WRITE_REQUEST_CODE);
        }
    } else {
        // Do not ask permissions in the runtime for API level 22 and lower
        finallySave();
    }
}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if (requestCode == STORAGE_WRITE_REQUEST_CODE) {
        if (grantResults.size == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            finallySave()
        } else {
            Toast.makeText(this, "Permission denied", Toast.LENGTH_SHORT).show()
        }
    }
}

private void finallySave(){
    try {
        File outFile = new File(imagesDir, "Abc " + System.currentTimeMillis() + ".png");
        FileOutputStream outputStream = new FileOutputStream(outFile);
        bitmaps[0].compress(Bitmap.CompressFormat.PNG, 100, outputStream);
        outputStream.flush();
        outputStream.close();
        scanFile(outFile, "image/png");

    } catch (Exception unused) {

    }
}

The problem is, sometimes the image is not saved and the following error is shown:

java.io.FileNotFoundException: /storage/emulated/0/Abc/Abc 1601370822977.png (Permission denied)

I found out that this error is thrown when the application is not asking "storage write" permissions on the runtime. Which is the very origin of the problem that I am facing. My app is not asking for runtime permissions in some cases despite I have written the code for it. Replicating this bug is also very difficult because many times the saving operation is carried out successfully.

I am sure that my code for saving images works fine because sometimes it works (this happens when my app asks for runtime permissions) and sometimes it doesn't.

P.S: I encountered this problem when I had reinstalled my app (after uninstalling).

EDIT: imagesDir is a global variable I am assigning is as follows:

imagesDir = getImageDir(); // in onCreate

private File getImageDir() {
    File directory = new File(
            Environment.getExternalStorageDirectory() + "/Abc");
    if (!directory.exists()) {
        if (!directory.mkdirs()) {
            Toast.makeText(this, "Could not create directory", Toast.LENGTH_SHORT).show();
            return null;
        }
    }
    scanFile(directory, "*/*");
    return directory;
}