PDPs are still being randomly removed from the DB.  From internal ticket, PEFTD-645:

  
I don't know a lot about this but I found the pap API for querying information about PDPs (https://ecomp-lis-509:30291/policy/pap/v1/pdps), and initially it returned this information (excerpt):
{
  "currentInstanceCount": 0,
  "desiredInstanceCount": 1,
  "pdpInstances": [],
  "pdpType": "xacml",
  "policies": [],
  "properties": {},

  ...

}

I looked in the pdpx error log and noticed some errors accessing the PDP-PAP topic during initialization and thought it might be due to that, so I restarted the pdpx-0 pod, and after it came back up, the same query returned:
{

  "currentInstanceCount": 1,
  "desiredInstanceCount": 1,
  "pdpInstances": [
  {
    "healthy": "HEALTHY",
    "instanceId": "pdpx-0",
    "pdpState": "ACTIVE"
  }
  ],
  "pdpType": "xacml",
  "policies": [],
  "properties": {},

  ...

}

And the db queries returned:

MariaDB [xacml]> select * from pdpsubgroup_pdp;
------------------------------------+++------------------------------------------------------------------------------------------
| pdpParentKeyName | pdpParentKeyVersion | pdpParentLocalName | pdpLocalName | parentLocalName | localName | parentKeyVersion | parentKeyName |
------------------------------------+++------------------------------------------------------------------------------------------
| defaultGroup | 0.0.0 | NULL | xacml | xacml | pdpx-0 | 0.0.0 | defaultGroup |
------------------------------------+++------------------------------------------------------------------------------------------
1 row in set (0.001 sec) MariaDB [xacml]> select * from pdpsubgroup;
-----------------------------------------+++----------------------------------------------------------
| CURRENTINSTANCECOUNT | DESIREDINSTANCECOUNT | parentLocalName | localName | parentKeyVersion | parentKeyName |
-----------------------------------------+++----------------------------------------------------------
| 0 | 1 | NULL | apex | 0.0.0 | defaultGroup |
| 0 | 1 | NULL | drools | 0.0.0 | defaultGroup |
| 1 | 1 | NULL | xacml | 0.0.0 | defaultGroup |
-----------------------------------------+++----------------------------------------------------------
3 rows in set (0.001 sec)MariaDB [xacml]> select * from pdp;
---------------+++-------------------------+-------------------------------------------
| HEALTHY | MESSAGE | PDPSTATE | parentLocalName | localName | parentKeyVersion | parentKeyName |
---------------+++-------------------------+-------------------------------------------
| 0 | NULL | 3 | xacml | pdpx-0 | 0.0.0 | defaultGroup |
---------------+++-------------------------+-------------------------------------------
1 row in set (0.101 sec)

But then after a few more minutes, the pdpx disappeared from db queries and the pap api output again.  So for some reason it is effectively getting deregistered.