Indeed, that is strange.
I did a debugging session on your site via FTP to see if I can find any traces of issues, but everything was normal, even so that WordPress did actually download the latest update itself, but after that it simply just failed.
Anyways, I think I managed to find a better resolution to this issue, which should work on your end as well. After the next update – probably after 4.20.5 the automatic updater should give you no issues.