Fluid app doesn't save cookies

I've got several Fluid apps for Amazon Web Services consoles (on different accounts). The aws console saves UI preferences between sessions (e.g., which columns in a display do I want to see). I believe but am not 100% sure that this is stored in a cookie. When I use firefox, this preference is saved between browser sessions. In the fluid app, it is not. I found the directory ~/Library/Application Support/Fluid/FluidApps/AWS-DEV Console. It has a cookies directory, which is empty. Other things are being successfully saved in this directory (Bookmarks, DownloadArchive, webhistory). At some point I discovered that Fluid was being sandboxed (I think; when I ran fluid apps i got a separate mounted pseudo-filesystem). The fluid apps had the finder attribute com.apple.quarantine set. I removed that with xattr and I no longer am in the sandbox. No effect on the cookies. Fluid Version 1.8.6 (2331). I have purchased a license key, which is installed. MacOS version 10.13.1 (17B48). Is this a bug, or am I doing something silly? ------ edit: It appears that the preference is saved in HTML5 local storage, not in a traditional cookie. I'm guessing fluid apps don't implement this.