I have had a support call in with Symantec and this is their answer:
We have received a confirmation from our developers.
In this case there is a hard coded time out value of 20 minutes.
Once you edit a filter, all member computer information is moved to a memory-based cache.
This time out value is NOT for console but in fact for the memory-based cache and after 20 minutes it is cleared.
To avoid losing all members of a static filter, please advise your colleagues to save it as soon as possible.