On the fly spell checking does not work properly

When activating on the fly spell checking, not all misspelled words are highlighted. Also in newly entered text, misspelled words are sometimes not highlighted. This behaviour is not really reproducible.

When "adding words to dictionary" via the context menu, this addition seems not permanent: When disabling and re-enabling on the fly spell-checking, those words are, again, highlighted. The context menu for spell-checking then offers the VERY SAME SPELLING as correction proposal.
After restarting Kate, added words are permanently lost. This behaviour is reproducible.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: kate 4:15.12.3-0ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-38.57-generic 4.4.19
Uname: Linux 4.4.0-38-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Sep 29 15:18:24 2016
DistributionChannelDescriptor:
 # This is a distribution channel descriptor
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-precise-amd64-20130203-1
ExecutablePath: /usr/bin/kate
InstallationDate: Installed on 2015-03-23 (556 days ago)
InstallationMedia: Ubuntu 12.04 "Precise" - Build amd64 LIVE Binary 20130203-13:50
SourcePackage: kate
UpgradeStatus: Upgraded to xenial on 2016-08-29 (31 days ago)