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I’ve been plagued by this Safari bug for many months. Every time I restart my mac, Safari does not remember my login to Gmail, Facebook, Reddit, Stackoverflow etc..
Finally, I found out what caused this seemingly deletion of my session cookies.
Developer mode
To fix, you have to disable it:
Safari > Preferences > Advanced > Show features for web developers > Uncheck
Safari is not very developer friendly, with this irritating long overdue bug.
I continue to use Safari as my default browser because of the ecosystem: Password manager and iPhone
But looks like when I need developer mode, Chrome will be my choice.