Librewolf, FirstPartyIsolation and cookies

My understanding of FirstPartyIsolation is that it places website data in a sandbox, preventing websites from reading data from other websites. If this is correct, then I would assume accepted cookies are still kept and not cleared when the session ends unless specified to do so.

I have Librewolf set to clear cookies on exit, however, a few sites such as EOS are whitelisted, and with FPI disabled the whitelist is respected, but with FPI enabled all cookies are cleared when the session ends.

Have I misunderstood FPI, or is the combination of FPI and some other preference causing havoc?