Some stored data remains after a temporary container is deleted.
All stored data should be removed.
Launch Firefox with new profile
Install TC
Configure TC to automatically isolate by domain and to delete empty containers immediately. Config file here: temporary_containers_preferences_2020-5-4_19.23.31.txt
Open Options tab, verify no data is stored from Yahoo:

Open new tab, load yahoo.com
Reload Options tab to see that data is now stored with Yahoo tab still open:

Close Yahoo tab. Notification from TC shows data being deleted:

Reload Options tab and see that some data remains:

Here is the debug log:
yahoo.log
Same thing happens with Youtube.
Can confirm that some storage remains on my end as well, at least on youtube:

Unfortunately that's not something that can be fixed on TCs end, since it just makes one API call to contextualIdentities.remove. So someone would need to report this to Firefox's Bugtracker.
There was a similar problem in the past: #127
Keep in mind, that even when the storage remains, it isn't accessible from websites since it's associated to the deleted container id.
I can verify that this happens with other container extensions as well, and on Dev 77.0b2 (64-bit) as well as Nightly 78.0a1.
Bug submitted to Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1636187
Hello,
I have the same issue as the OP but found a way to delete those empty cookies (I don't even know how to call them) upon closing the browser.


It's not an elegant solution for those who want to keep track of their history, but thought I would share it anyway.
P.S. I have the Cookies option unchecked because I use Multi-Account Containers to keep cookies from certain websites.
Most helpful comment
Bug submitted to Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1636187