Interestingly, I encountered an issue with Auto-login last week that I forgot to report. This may have been related to the machine.
I performed a clean Windows 10 installation on a retired 2011 Sony VAIO (2nd Gen i5-2410m, 8GB RAM, new SSD). The machine was so painfully slow, it was taking 1M45s to shutdown and 2M from zero up to the Windows Desktop. I installed Firefox, set the homepage to
https://mxlinux.org/, re-timed it and the the extra step took the time from 2M to 2M33s, tested 3 times to confirm.
Thinking this would be a great machine to demonstrate how MX can revitalize an older system, I partitioned off 40GB and booted a freshly made MX-23.3_KDE Live USB. After installing all the updates, I ran the installer then rebooted without the USB key. Having to enter the user PW I realised I had forgotten to set the auto-login option during installation, so using MXUM, I set the user to auto-login, then using Session and Startup I added Firefox to which I had also set to load the MX Homepage to load.
On rebooting, the user acct did not auto-login and checking all the settings, nothing I did at the GUI level was going to make that happen. As per the OP's issue, autologin was not working. I didn't have time to waste so rebooted live, repeated all my earlier steps and ran the installer, selecting auto-login and all went well.
All this to say I have seen first hand what
@Johyn is reporting, but I have no evidence to produce. I was recording the whole thing as a time lapse operation at 15x speed with an old phone running a clock timer next to the laptop, but when the process turned into a failure, I deleted the recording. Even if I hadn't, the fine details would have been missed with 4s worth of playback equalling 1M of real time.
I can't go this far without revealing the result of my testing, that would be cruel. The bootup time from grub to FF opening the MX Homepage was 32s with SysVinit, and 36s with SystemD. Shutdown times are 11s with SysVinit, and 2s with SystemD, so significantly faster than Windows10. What is not stated is all the timings here were
after my first MX Dual-boot installation, where I could have a fixed and consistent start time for each boot, a Grub OS entry selected and a keyboard tap to start the timer on each startup option.