Do you have a relevance condition on the nested repeat? It looks a bit like https://github.com/enketo/enketo-core/issues/938 If you could share your form either in this thread or in a direct message that would help us further troubleshoot.
My guess and hope is that it's only not visible. A quick way you can verify this is by going to the submission detail page (https://docs.getodk.org/central-submissions/#submission-details). The address should look something like
https://<server>/#/projects/<projectid>/forms/<formid>/submissions/<submissionid>
If you change the # to a v1 and add .xml to the end, you'll be able to see the raw submission that Central received. If the information you expect is there, that confirms it's a display issue.