As reported here:

https://mail.openjdk.java.net/pipermail/zgc-dev/2019-September/000736.html

The ZDirector heuristics can get of to a bad start if the statistics is contaminated by early "Metaspace GC Threshold" GC requests. To avoid this, we could simply ignore such requests until the GC is warm, at the potential cost of expanding metaspace a bit more during startup.