Please be aware you may see different period key formats on the VAT DETAILS page than you are expecting. Many will have alpha numeric period keys (e.g. 18A1) and will not have any open obligations for periods prior to joining MTD. However, if you have a fully numeric period key (e.g. 0419) you will be able to see your open obligations for periods both before they have signed up to MTD and since joining MTD.
In some cases this shows these numeric period keys as open, despite the VAT return having been submitted and having received a success submission message. HMRC are working to update these obligations at the earliest opportunity. You should not attempt to make a further submission for a period already submitted, as this will not resolve the issue. HMRC are aware of some instances, where the obligation for a numeric period key is not viewable and they are urgently reviewing the issue and will provide us with a full response as soon as our investigation is complete.