Backdating submissions is not saving correct dates anymore

The problem:
The first two issues with backdated submissions saved with a backdate for each paper. Subsequent submissions have not saved the backdate. The backdate saves for Issues but not for papers. It shows in the correct old date in the date published box but when I click Save, the box empties. Page numbers are saved but not backdate. Each paper shows with the current date as a publication date.
*In some of the earlier submissions, the re-entered date holds but in later submissions, the date is replaced by the current date.

I tried to re-enter the date by unpublishing etc but this has not worked.
For example: See the first and subsequent issues here: https://humansystemsjournal.com

Using OJS 3.4.0.6