I don't use sshd so I don't know where its configuration file is other than it'll be under /etc/ somewhere.
When an rpm installs an application that has configuration files it also has to install default configuration files. However, it is usually programmed to check whether the files exist first. In the case of an upgrade or reinstall the files will already exist and may well contain a wanted configuration. When this happens there are two options: either the rpm will save the default files alongside the live ones with the suffix .rpmnew, leaving the live ones unchanged, or it will rename the live ones to...rpmsave and install the default ones as the new live files. Either way it will usually tell you which it has done as output, which Synaptic will display in a dialogue box.
The general wisdom on this forum is that such output can safely be ignored, which is true if you're just using default settings for everything. On the other hand, if you've configured it you want your own settings back. However, it's not quite as simple as making sure you have your original file in place, because the whole point of an upgrade is it has replaced the application with an (hopefully) improved version, and there might be some changes to the configuration options. So I would always advise reading through both the old and new files and merging any new options into your old file or merging your changes into the new file, whichever is easier, and then making the merged file the live one.
There are supposed to be automated ways to do this but I don't trust them because I've seen them make a terrible mess in the past. So I just do it by hand.
I don't know whether there's been an upgrade to ssh, but if upgrading the system returned it to default it's the most likely explanation.