Unmark Program as Recorded

The command:
curl -XDELETE "http://192.168.0.212:8089/dvr/programs/EP001887106397"

Always returns:
true
0

Could you return "false" if the command failed to do anything?

It just means your command had the correct syntax.

Doubt it. You're using an internal, private, undocumented, unsupported API.
Check /dvr/programs to see if the entry was actually deleted.
It should be deleted, until you pause a pass that records that episode, then it reflects the recording you still have. Acknowledged behavior - Recorded State Overwritten when Pass is Paused - #18 by eric