I am afraid its a new issue.
Update the SKU value on main variable product is not shown, after successful update message.
While, update of each SKU value variation of this product succeeds.
I am using WooCommerce (4.8.0) WordPress (5.6)
Turned off all plugins, but Woocommerce - did not help.
Replaced theme to default 2020 - did not help.
Debug is ON and clean on all tests & No JS issues reported.
Any idea what might be the issue? or how to debug it?
Does your variation have a ton of variations? If so it's possible that not all fields are saved due to too many form fields being submitted at once.
Also see: https://docs.woocommerce.com/document/problems-with-large-amounts-of-data-not-saving-variations-rates-etc/#php-539-and-max_input_vars
You can edit your php.ini to allow more form-fields: