Note: This behaviour is expected.
In the case of a Sandbox being created from a production instance, where the airSlate app has already been installed, you may encounter the following issues:
custom buttons that have been created in production environment don’t display in the Custom buttons tab of Admin Tools
custom buttons that have been created in production environment fail when using them on layouts or lists
To solve this issue it is required to create a custom button from scratch in the Custom buttons tab of Admin Tools.