open a finder window with quarter size of screen, snap to top for maximise window then grab and drag title bar to bottom to restore window size to replicate this
yep, the problem is the macOS dragging API which keeps it's own track of the window positions.
BTT first correctly places thr window, but as soon as the next drag event comes in, macOS restores the position it thinks is "correct".
This bug was introduced in macOS 10.12 and only happens when grabbing a window more on the right half of the screen as far as I know.