Svn will not remember passphrase for client certificate.

Subversion v1.9.3, Apache2 v2.4.18

Our subversion server uses SSL with client certificates. When performing
any action against the repository using the svn client, a prompt appears
asking for the password to the client certificate. (Note: this is not the
user id / password combination).

Expected action: After the first time prompting for the password, svn
should have preserved it and not asked for it again.

Below is a sample session. Attached is the user's config file.

Any help you can offer will be appreciated.
Thanks.

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cc_at_ccvm:~/Robust/Client$ svn --version
svn, version 1.9.3 (r1718519)
   compiled Mar 14 2016, 11:39:40 on x86_64-pc-linux-gnu

Copyright (C) 2015 The Apache Software Foundation.
This software consists of contributions made by many people;
see the NOTICE file for more information.
Subversion is open source software, see http://subversion.apache.org/

The following repository access (RA) modules are available:

* ra_svn : Module for accessing a repository using the svn network protocol.
  - with Cyrus SASL authentication
  - handles 'svn' scheme
* ra_local : Module for accessing a repository on local disk.
  - handles 'file' scheme
* ra_serf : Module for accessing a repository via WebDAV protocol using
serf.
  - using serf 1.3.8 (compiled with 1.3.8)
  - handles 'http' scheme
  - handles 'https' scheme

The following authentication credential caches are available:

* Plaintext cache in /home/cc/.subversion
* Gnome Keyring
* GPG-Agent
* KWallet (KDE)

cc_at_ccvm:~/Robust/Client$ svn update
Updating '.':
Passphrase for '/home/cc/Robust/ccapon.p12': ***************

At revision 3127.
cc_at_ccvm:~/Robust/Client$ svn update
Updating '.':
Passphrase for '/home/cc/Robust/ccapon.p12': ***************

At revision 3127.
cc_at_ccvm:~/Robust/Client$