For anyone interested, they left a long and informative comment there about the bug, summarized below, and they closed the issue. You can still vote for the issue and subscribe if you want to be notified of any update (I suppose they’ll update it when there’s progress).
A new ticket has been opened for Windows (49300), and there’s one for Linux (39758).
When it happens
Some folks report when closing. Others report it happens w/ browser open after a certain amount of time.
Things that folks have asked to check
These are things which could be giving the same symptom but it’s a different cause.
Is “Forget me when I close this site” enabled for site being visited? (click Brave logo next to omnibox)
There is also a global “Forget me when I close this site” located at brave://settings/shields
Make sure 3rd party software isn’t tampering with cookies / etc (virus scanner, etc).
Many people who are experiencing this issue and have responded here have checked those things and they’re not applicable.
It’s possible Google/Chrome has a value set in their Finch server which we are missing and need to set too. Or they might have a different code-path and we might need to disable a specific feature. We’ll keep digging in to see if this is the case. We can make changes here without making a new binary.
Additional technical information
There are some browser events that happen which are notable. For example, on startup and every 1 hour, the browser itself will check for updates to components (adblock list, extensions, etc). This type of thing (checking at start up and periodically on an interval) happens for other values too. Chromium browsers (ex: Brave) each have a variations server (aka experiments system) configured. Google Chrome uses a system called Finch and Brave uses one called Griffin. Microsoft has one too which they also share and use for MS Office and other products.
We are actively monitoring the values Google sets in Finch and watching for multiple situations. One situation we watch for is when Chromium is having a feature ENABLED by default in the code but then it’s OFF for 100% of users in Finch. This particular situation has bit us before.
My guess is that there is a feature enabled (or disabled) in the code which is causing a code-branch for some people where the session termination happens prematurely.