Yes, plural. Three of those dang things showed up in my mob farm. According to the book, they're only supposed to spawn like a million blocks away from spawn. My base is a few thousand away from spawn in a desert. For now, I've turned off farseer spawns in the config.

So this farm consists of dreadful dirt from Mob Grinding Utils. Mobs spawn on the dirt when the light level is five and below. Everything was going fine until I heard this deep growling sound but saw nothing in the farm or around me OR on my minimap (Xaero's). I hopped into Creative thinking something glitched/bugged out. Still didn't see anything. Then I went in Spectator and saw THREE farseers in my mob farm. They were invisible and could only be seen while in Spectator mode. I assumed farseers could only spawn near world borders according to the in-game book but oh boy was that wrong. Is this a bug or intended behavior for the farseer? And do let me know if the issue is not on your end and I'll mosey on over to the other mod's author.

1.19.2-forge-43.1.43
mob_grinding_utils-1.19.2-0.4.46
alexsmobs-1.21.0

P.S. Terrifying experience for someone in diamond-equivalent gear and no enchants with only dark glass and a box of sandstone between them and certain death. I'd rather get slapped by a komodo dragon... again.