Keycloak seems to ignore cache property `purge` in configuration

Since I got no reply to my other post, I tried to get the keycloak cache persisted by writing to file.

I see that the files get created on startup and Keycloak writes to file when I log in. However the files get truncated whenever Keycloak restarts. There is a property purge that is described as “If true, purges this cache store when it starts up”, but setting this to false seems to have no effect or is ignored.

I have this configuration for all distributed caches (sessions, actionTokens, etc.):
and I also tried setting fetch-state and preload, without any effect.
As soon as Keycloak starts the files get truncated.