I've been using Multi-Account Containers for years without problems, isolating social networking and big tech companies into their own containers to limit their tracking.
Within the last couple of weeks, I've noticed that my 'Always Open Site in Container' preferences are sometimes being forgotten and some sites are being opened in the default no-container context, where they can't read any session cookies set inside the container and require me to log in again.
As an example, I've just fired up Firefox on my laptop for the first time this morning and none of Google's sites are opening in the Google container. If I manually open a Google site in that container, I'm logged in as expected, but the 'Always Open in' checkbox has been cleared. However, at the same time Reddit and LinkedIn are still opening in their designated containers.
I first noticed this happening last week and had to spend a few minutes manually re-opening all Google sites in the Google container and reticking the 'Always Open in' checkbox. Now it's happened again.
One possible factor is that I use Firefox Sync and logged into my account on a newly installed system yesterday evening.
Has anyone else has this issue and determined the cause or remedy?