PySide6 QPlainTextEdit sometimes don't show all letters

Question

As you can see in the image below sometimes some letters don't show in text box or they might show earlier when the textbox is updated with other characters.

The only thing I do with the text is to use the function appendPlainText(text) with a certain message.

Does anyone know how to solve this issue ? If you need any code let me know.

Edit (Add code):

I have a file which runs the code for the study selected from the combo box which uses a log function that is also in another file.

utils.py

LOGGING_OPTIONS = {
    "level": logging.INFO,
    "format": "<%(asctime)s | [%(levelname)s] | %(message)s",
    "format_log_container": "<%(asctime)s> %(message)s",
    "datefmt": "%H:%M:%S",
    "handlers": [logging.StreamHandler()],
}

logging.basicConfig(
    level=LOGGING_OPTIONS["level"],
    format=LOGGING_OPTIONS["format"],
    handlers=LOGGING_OPTIONS["handlers"],
)
logger = logging.getLogger()

def log(message, level=logging.INFO):
    if level == logging.INFO:
        logger.info(f"{message}")
    elif level == logging.ERROR:
        logger.error(f"{message}")

def delete_sheets(workbook, file_path, sheet_names):
    for sheet_name in sheet_names:
        if sheet_name in workbook.sheetnames:
            log(f"Deleting sheet: {sheet_name}")
            workbook.remove(workbook[sheet_name])

    workbook.save(file_path)

This log function is used everywhere when I want to log something.

dta_302.py - file with study code

def main_302(file_path):
  try:
      dta_sheet_names = [] # I get these names from an object
      workbook = load_workbook(file_path)
      delete_sheets(workbook, file_path, dta_sheet_names) # Is in utils file
  except IndexError:
      log("Could not load the file. Please introduce the correct path.")
  except Exception as error:
      log(error, logging.ERROR)

gui.py - file that manages the gui

class QtLogHandler(logging.Handler):
    def __init__(self, widget):
        super(QtLogHandler, self).__init__()
        self.widget = widget

    def emit(self, record):
        msg = self.format(record)
        self.widget.appendPlainText(msg)


class Signals(QObject):
    started = Signal()
    finished = Signal()


class Runnable(QRunnable):
    def __init__(self, func=None, *args):
        super(Runnable, self).__init__()
        self.func = func
        self.args = args
        self.signals = Signals()

    @Slot()
    def run(self):
        if self.func:
            self.func(*self.args)
            self.signals.finished.emit()

class MainWindow(QMainWindow):
    def __init__(self):
        super(MainWindow, self).__init__()
        studies = [""] # Options for combo box

        self.file_path = ""
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)

        # logger handler
        self.logger_handler = QtLogHandler(self.ui.logTextContainer)
        self.logger_handler.setFormatter(
            logging.Formatter(
                LOGGING_OPTIONS["format_log_container"], LOGGING_OPTIONS["datefmt"]
            )
        )
        utils_logger.addHandler(self.logger_handler)

        # Process to read console output
        self.process = QProcess()
        self.process.readyReadStandardOutput.connect(self.handle_stdout)  # type: ignore
        self.process.readyReadStandardError.connect(self.handle_stderr)  # type: ignore

        self.ui.selectStudyComboBox.addItems(studies)

        self.ui.selectFileBtn.clicked.connect((self.select_file))  # type: ignore
        self.ui.submitBtn.setEnabled(False)
        self.ui.submitBtn.clicked.connect((self.start_submit))  # type: ignore

        # Worker
        self.threadpool = QThreadPool()
        self.worker = Runnable(self.submit)
        self.worker.setAutoDelete(False)
        self.worker.signals.finished.connect(self.restore_ui)

    def add_log_message(self, message):
        self.logger_handler.widget.appendPlainText(message)
        self.logger_handler.widget.ensureCursorVisible()

    def handle_stdout(self):
        message = self.process.readAllStandardOutput().data().decode()
        self.add_log_message(message.strip())

    def handle_stderr(self):
        message = self.process.readAllStandardError().data().decode()
        self.add_log_message(message.strip())

    def select_file(self):
        file_dialog = QFileDialog.getOpenFileName(
            self, filter="Excel Files (*.xls *.xlsx *.xlsm)"
        )

        self.file_path = file_dialog[0]
        self.ui.selectedFilePath.setText(file_dialog[0])

        if self.file_path:
            self.ui.submitBtn.setEnabled(True)

    def start_submit(self):
        self.ui.selectStudyComboBox.setEnabled(False)
        self.ui.selectFileBtn.setEnabled(False)
        self.ui.submitBtn.setEnabled(False)
        self.threadpool.start(self.worker)

    def submit(self):
        selected_study = self.ui.selectStudyComboBox.currentText()

        if selected_study == "LC-FAOD 302":
            start_time = time.time()
            main_302(self.file_path)
            end_time = time.time()
            log(f"Running time: {end_time - start_time} seconds")

    def restore_ui(self):
        self.ui.selectStudyComboBox.setEnabled(True)
        self.ui.selectFileBtn.setEnabled(True)
        self.ui.submitBtn.setEnabled(True)
        self.add_log_message("")