WordPress treats the Child Theme as a separate theme and hence the customizer settings will be fresh while switching to the child theme. So, it would appear that you lost the changes you made in the customizer. However, the settings remain fine in the parent theme’s settings i.e. if you switch to the parent theme, you’ll find the changes you made fine.

You can transfer all the customizer settings from the parent theme to the child theme. Follow the instruction given below

Activate the parent theme if you have not already.
Install/Activate the Plugin to Export/Import Customizer Settings
Go to WordPress Dashboard > Customizer > Export/Import
Export the customizer settings by clicking the Export button there.
Now, switch to the child theme from WordPress Dashboard > Themes, or from the customizer, as shown in the video below.
Again, go to WordPress Dashboard > Customizer > Export/Import
This time, click Choose File button and select the previously import file
Click Import button

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