Once particular sandbox is published properly, changes made in that sandbox will be merged with mainline metadata.
Creating some more changes in another new sandbox and publishing the same will not overwrite the changes of earlier sandbox if it is published properly.
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Customer should be facing issue here because of one of following reasons...
1) Sandbox2 is created before publishing Sandbox1. Sandbox 2 must be created after Sandobx1 is published.
2) While publishing the particular sandbox, Layer Name and Layer Values are not set to 'all'. Doc give by customer does not include screenshot of 'Publish Sandbox'.
3) Once user clicks on 'Customize Pages', 'SITE' layer should be chosen in both of the sandboxes. If 'Country' or 'Organization' layer is chosen for editing purpose, Value of this layer should be same in both of the sandboxes. …show more content…
This can be checked via 'Manage Customization'. Here, we can provide file path and check if customization is available at different layers.
Support, Please help customer to check above things and ensure that they're following steps properly.