Oct 1 2020
What I mean is that there might be some problems with the scripts (or something) which will be loaded in the very end of the editing page loading procedure, and if the script is not loaded, and the user successfully submitted their edit, the local cookie would stop the problem from occuring for some time.
@Urbanecm It might be a bit late to tell, but earlier this year, the local community have noticed that if the users who run into the problem submit their edits before the 2010 version of the wikitext editor load completely, the submission would be succeeded and their sessions would not lose in a few days. That might be a clue about why it happens.
Apr 25 2020
Mar 20 2020
Feb 25 2020
Additionally, the users who lost login session reported that their session won't lose if they are using mobile view to edit.
Though according to the test, the problem has been fixed for some reason, on Feb. 25th, still, user RavenclawOIer reported they are losing login session for no reason.
Feb 24 2020
As Tigerzeng requested, I'm posting the test result here.
May 7 2019
The new fixing do fixed some brackets. But, unfortunately, the edit summary, the page size changing and the edit tag should be covered with brackets too.