Hi Tom,
Thanks for the form and files. I tested it locally and the Inbox appears to work just fine. As your repeat is associated with the pulldata function with the external csv file, I might be that the pulldata function was not working properly with the existing feature service. We recently introduce the new implemetation with Inbox setting in both Connect and field app in 3.10 beta, you might want to give it a try with 3.10 to publish the servey and check the inbox in 3.10 field app as well.
Please check this documentation on EAC and use 3.10 to give a try.
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If the missing Inbox records still occurs using 3.10 beta, I would probably try to create a new copy of survey and use the existing feature service, adding in the csv files and republish the survey as a workaround.