TL:DR: After an update, make sure to increase the number of replays WOWS saves to what you previously selected (and/or back up your replays to another folder) BEFORE starting the client.
As many of you know WOWS has now enabled saving replays by default WITH PATCH 0.7.11. While this is a welcome change, there is the limitation that the client defaults to saving only the last 30 replays. I imagine this is to help limit the amount of space used by game components which is a good thing, but disastrous if you like to keep all of your replays.
To increase the number of replays saved, you can either make a simple change to a file or do it through the Settings on mxstat (the method I use):