I can't upgrade from 3.0.3 due to data loss.

Did you mark the sub-fields as translatable? That feature was actually released in 3.0.3. Other than that, you may want to revisit the field settings and save again. I don't know otherwise why you would be experiencing this.

I'm pretty sure the issue is a configuration issue in the field settings. Please verify that the custom field and the sub-fields (you want translatable) are marked as such. Thats really the only explanation as to why you're not seeing those fields in the form but there no updates that would have wiped out your data. You're just not seeing the values in the form from what i can tell from your screenshots and without those subfields being translatable, they are going to receive the default language value on save.

Perfect, it was what you said, it was just necessary to activate the translation in the subfields of the custom field.

Thanks!