I'm playing on PC via Steam and have had this happen on both 1.1-1198 and 1.1-1221. You can see in the upper right corner the dates and times these things happened. The first time I assumed it was a glitch caused by the Winter Fair breaking pathfinding, but now Walter's doing it, too.
#NPCs keep spawning in my pasture
1 messages · Page 1 of 1 (latest)
dang! you got a visit from The Jeff Smith?? 

Oh! Maybe they want to have a party at your farm, guppygirl! I want to be there too. lol 
Do you happen to still have the save or backup file on that day?
If you're not progressing too far, kindly share your Backup Folder with us. 😊
I don't know if it's relevant, but the time I had several characters there I watched both Zarah and Chaem spawn in behind the cheese press before joining everyone else in the corner. It's 2 am for me now, I'll see if I can get the backup save tomorrow.
I see! I'll add that to the note. Yes, yes. Kindly send it whenever you can.
Have a good rest! 
Unfortunately, it seems like I've gone too far for the backup to still be there - farthest back is goes is Spring 5 Year 4. If it happens again I'll post the save immediately.
Actually! I reported a bug where NPCs were wearing their Spring outfits during Festival minigames and uploaded my save file there on Winter 28 - the same day Walter spawned in my pasture. You should be able to use that save file to see if you can recreate that bug!
Ahh I see! I'll check that file with the team. Did Walter appear on your Farm around 10 AM like in the first screenshot on this post?
Correct!
Got it! We'll be checking on it, and if the issue reappears again in your gameplay, kindly reach out to us. Thank you, guppygirl!
Winter 28, Year 4... Walter is in my pasture again.
Wow! You're progressing so fast, guppygirl.! Thank you for keeping us updated.
Our team is still working on the fix now. Thank you! 
Walter wanted to visit me too, it seems (he's the first one being lost on my farm xD)
He walked in from the left at around 8am and then just stood there next to the upper right pond
However he was at the new years eve festival that same evening, so that worked.
Hello Brosinity! Thank you for adding your report here and I'm sorry for the delayed response. I'll add your report to the team. Thanks again! 
Same thing is happening to me with walter. Its been happening all (game) day
The NPCs kept spawning by my coop for one night only - it stopped after that. Xbox/PC Xbox. I did get video captures, but it has been 3 in-game seasons since it happened, so I'm guessing it's a one time thing for me.