Thanks @Kal_Lam - we’re using a kobo-hosted server (but not the UNOCHA or Kobotoolbox ones) - will DM you with the video I have been sent.
The XLSForm has a large number of repeat groups - based on the one here:
Problems with data in generated repeat groups in Enketo It validates fine on the ODK XLSForm validator and hasn’t had problems in test (or for most users in Prod). Many thanks for taking a look!