If you installed a UCS from v9.4.4 or v9.4.3 onto this unit at some point then it's highly likely this is CR 108866.
Unfortunately there's no publically downloadable resolution for this CR at the moment, but if you contact F5 Support we can make available a HotFix for v9.4.5.
Sadly, in order to resolve this, you need to install the HotFix
before you load the UCS from the earlier version - so at the stage you have found yourself the only fix is to (as you have done) load the signatures again manually and then manually create signature sets as needed.
To have a full fix you would need the HotFix (or the next release of the software, once it is available), a clean installation and then go ahead and re-load your v9.4.4/3 UCS.
There will be a solution published about this shortly on AskF5.
Aaron