Losing theme customization when switching from a theme to a child in WordPress

Not sure about "lost" header. Was it part of markup? CSS background? Custom background?

Losing menu assignments isrmal behavior.

• Child theme is not same theme as parent

• It is completely separate theme, that uses parent as fall-back

Child theme is loaded and processed before parent theme (yeah, seems a little backwards). Don expect it to act simply as parent theme with some changes. It seems like that, butt quite that internally.