I have been having this issue with my last two households and I figured out that the culprit is a random NPC in the neighborhood that happens to have an ailment (usually Gnome Lure) causing the issue. It doesnt matter if the NPC knows you, or has entered your house.
At first I thought it was because my previous Sim was in the Naturopath career and also lived in Innisgreen, so maybe it was more prone to ailments and gnomes. However, it was happening so frequently and after verifying it wasnt an Eco thing, or a lot trait, I went shift + clicking on every NPC in the neighborhood, checking their listed moodlets until I found the one with an existing ailment. I would remove it and I would be gnome free for a while until another NPC had it. Almost all NPCs affected were not known to my Sim.
I then started a new game, new household and my sim currently lives in Ciudad Enamorada. Random gnomes started appearing in her house, despite not having any guests over and not being into anything Naturopath related. I already figured it was an NPC but only figured out who it was, because after completing the relevant quests, Sammy the Bear turned back into a Sim, and he had ailment spots on his face, lol. Sure enough, I checked his moodlets and he had the gnome ailment.
Either the gnome ailment itself needs to be modified, or the criteria for NPCs to have an ailment should be modified. Hope this helps! I tested this with and without mods.
