That was in fact a feature of infinity for reddit, but the multireddits were a reddit thing( infinity just made it easy to have access to it and manage them in some clever ways)
Now in regards of eternity, this was already questioned on the old community here: https://lemmy.world/post/3613830
Tl:dr the dev probably will make it a feature in the future, but for now it’s not working
Hummm, do you have or had a display name? By had I mean when you logged in eternity did you have one, even if you removed afterwords.
This just reminded me of this other post and the problem at that time seemed to be with display names.
But if that wasn’t the case, did you try the basics? Log out and in again, clear app cache, uninstall and reinstall the app, check if it is the latest version and any info if that is also happening in the nightly build?(turns out the basics are pretty long) nonetheless any more info can help.