Please notice that there are many sellers (ebay, aliexpress, ..) who sell mfrc522 boards. The quality of these boards are extremely different. Some are soldered with wrong/low quality capacitors or fake/defect mfrc522.
(2) SPI Loopback Test using Python (See Appendix C for the python program) – rpi.stackexchange 2019
(3) SPI Loopback Test (struct spi_ioc_transfer level) – rpi.org forum 2019
So as I said earlier, I will forget SimpleMFRC522 and directly use Mario Gómez’s original version to learn what he is doing. He also explains about the requirements of SPI-Py and GPIO, both of which I suspect is not standard, and might not be compatible to Rpi4B buster. But I am only 50% sure.