I have them working in the Smiths FMS by adding that isg_ prefix (I think the _awy also file needed renaming to something else) and so far it works very well. Although all I have tried is programing the FMS on the ground, I've not tried flying the sim with it yet... I have also NOT tested whether the 'full' data can be read - ie intersection XANUT etc.
One assumes that either Navigraph have mis-named the files (unlikely) or that there will be an update to the ISG1 gauges to fix various issues and to read the newly named nav data... (changing the names of the data files at the same time as an update is actually quite neat as it ensures that users update their setups)
Now, looking forward to that update

...
EDIT. With the new data XANUT, VAVOS, VEKUV etc are not available as DIRECT waypoints as before (this is not a surprise since the gauges are not updated yet)
Geoff