Hmm it’s an interesting thought, but certainly nothing I know I installed. I thought maybe the duckduckgo privacy manager plugin might be a culprit, but I’m not finding any settings to auto-clean cookies. Checking my cookies, I have some going back 8 months (the last time I did actually clean them up) for sites I haven’t visited since then, so I’m not sure why only the cookies from Lemmy would be getting erased?