Overview of the Issue
When editing a product translation using the standard product editor, existing translations of custom attributes are being deleted.
Workaround
This issue is fixed in WooCommerce Multilingual 3.6.5. Updating solves it on your site, so you only need this workaround if you can’t update yet.
Use the WooCommerce Multilingual products editor to edit products translations or download WooCommerce Multilingual 3.6.5-beta1 from the WPML.org downloads page.