I don’t have sebbies so I can’t really say if that’s the cause of the wavy feathers.
What I was referring to are these. Stress bars.
View attachment 4170841
Stress bars represent a brief halt in the feather shaft growth where there either wasn’t enough nutrients for proper feather development or there was some other stressor.
The problematic thing about stress bars is that it shows that there
was an issue but not necessarily that there’s still an issue. So it could have been something to do with the gosling going off feed at the time it was introduced to the rest of the flock. The stress bars will remain until the next molt.
You can add a multivitamin to their water but I would keep an eye on wether the gosling is eating enough, watching to see if they’re being chased away from the feed by the other geese and watch for other signs of a nutritional deficiency, such as excessively eating dirt or sand, limping, drunken gait, depression, excessive growth of nails or tomia “the teeth like serrations in the mouth.”