Create a Display Suite Custom Block Field Programatically
As already reported in this DS issue, you need to alter DS_BLOCK_CONTENT to DS_BLOCK_TEMPLATE.

We had a similar issue where from a certain point in time, the block was no longer rendered.
That point in time was a security update on the DS module, we applied somewhere a month ago : 
https://www.drupal.org/sa-contrib-2018-019