For anyone having the same issue: We were not able to identify why this happens. We "solved" this by not relying on blob/content uris for the mobile version of our application at all. Instead, we use react-native-blob-util to copy the attachment to the applications cache and get a path for it.

At certain points in the life cycle, when we know we don't need the attachment anymore, we delete it again, also using react-native-blob-util.