(Lemmy regression) opening a new tab loses session cookie, thus logged out; update: hard-refresh also logs you out

In the Lemmy web client it used to be possible to open a new tab (control-tab) which would naturally be logged in. That goes for most websites. With Lemmy it started getting flakey (sometimes works, sometimes not). Lately it’s working less often and it seems browser flavor is a factor. Tor Browser (FF) generally works, but Ungoogled Chromium new tabs are logged out. So in UC, I have to do everything for a Lemmy instance under one tab.

I wonder what kind of funny business causes session cookies to fail. My guess is they are not using session cookies for logins but rather one of the rare alternatives.

update

With just one tab running, I did a hard refresh (control-shift-R). That logged me out presumably doing the same as getting a new tab. Using the /back/ button does not recover from this.