I just experienced what seems to be the same issue with Sebastian on iOS 1.6.15.1 (Build 25156).
This happened on Fall 12, Year 8, in the evening. Sebastian walked from the room into the dark/out-of-bounds area on the left side of the farmhouse. He then came back through the wall into the room, but shortly afterwards he walked through the upper wall and went even further into the dark area until he disappeared from view.
Eventually, he somehow returned to the bed and went to sleep normally.
I was able to capture both screenshots and a video of the behavior.
Interestingly, I also saw Sebastian do something very similar once before in the morning, although I only managed to take screenshots that time. So far, I have seen this happen more than once.
Video and screenshots attached.
It was quite surreal watching my husband casually walk through the walls and disappear into the void.
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↓This screenshot shows the first time Sebastian passed through the wall.
Game version: 1.6.15.1 build 25156
Device: iPhone 17
Single player, no mods.