There were changes to Lithium restconf, and after OPNFLWPLUG-378 was fixed this one become visible.

Using add-lsp call from bgpcep project https://wiki.opendaylight.org/view/BGP_LS_PCEP:Programmer_Guide#Creating_LSP_2
on older Lithium (and Helium) builds was returning
JSON:
{"output":{}}
XML:
<?xml version="1.0" encoding="UTF-8" standalone="no"?><output xmlns="urn:opendaylight:params:xml:ns:yang:topology:pcep"/>

But currently:
JSON:
{}
XML:
<zdef648384929:data xmlns:zdef648384929="urn:ietf:params:xml:ns:netconf:base:1.0"></zdef648384929:data>

There was no significant change in PCEP code, so this is a resconf issue and https://tools.ietf.org/html/draft-bierman-netconf-restconf-02#section-5.4.2
implies this is a bug, not a feature.