The cache will try to maintain 5 GB free on the cache volume at all times.
If you write data to the drive, at 6 GB free space, your writes will begin to get throttle. At 5 GB free space, stage 2 throttling kicks in (which is even more aggressive), and the cache begins to trim to make room for more writes.
Other than that, there have been a few breaking changes in the cache data structure format which required the cache to be reset upon upgrading to a newer version. For example, the cache now aligns itself to the NTFS compression unit size and that alignment means that the cache needed to be reset.
A system crash will maintain the cache, but it will reset the learned usage data.