Hello,
We tested the creation of the page with the custom sidebar option and did not face issues with the auto-copying of the sidebar position. Do you use Translate and duplicate option to create translation https://gyazo.com/728af0f4626b65e4ae978e3e707d5740
Make sure that you enabled “Copy” for the sidebar in the WPML settings https://gyazo.com/0fa896968f0bcae6dee594369de2098c
Provide us with WP Dashboard access to your site to test the issue if it does not help, please.
Regards