Thats expectable. You are keep capturing the building with 3 strong units. So every point they decay you are recapturing. Your friend could have do the same. In fact, he did with some but I they were common pikemen, so not enough to keep control up.
So it’s not a bug or an exploit. He would have kept the buildings if he had used champions.
EDIT: @dinuruian i updated the video, it was bad cropped
20260310-1207-35.7648851.mp4
@dinuruian I just watched the replay.
When you captured the barracks, some of your pikemen kept capturing the building, while your friend didnt do the same when recaptured.
Thats why you kept the control of the building and he didnt