Hello,
Take into account that when you switching themes widgets could be messed up that’s why after switching you need to go to Appearance > Widgets > and place them to correct areas.
Also, child theme inherits parent theme styles but it does not inherit theme options, so if you want to switch theme just for testing purposes then just ignore the missing header but if you want to switch to other theme on regular basis then don’t forget to Export theme settings before switching (Theme Options > Export/Import ) and import them after switching.
Regards