Background of the issue:
I created a simple ACF block with the setting 'Same fields across languages'. I added three fields to that block: 'Button Group', 'Group', and 'Button Group' inside the group. All field names were unique, and I tested this on a clean installation.
Symptoms:
'Button Group' inside the 'Group' field behaves differently than expected: it appears in the Advanced Translation Editor.
Thank you for contacting WPML support. I'd be happy to assist you on this issue.
I have created a public test site with ACF pro and WPML here, would you mind exporting the field group you created on your site here so I can check this further?
Thank you for your report. I have been able to replicate the issue and have escalated it to our second-tier support team. I will update you as soon as I have any feedback available.