I suspect you need to pin the elements to avoid their expiration. Try config that looks something like this:
<cache name="myBigMemoryGoStore"
maxBytesLocalHeap="1M"
eternal="true"
maxBytesLocalOffHeap="2G">
<persistence strategy="none"/>
<pinning store="inCache" />
<sizeOfPolicy maxDepth="100000" maxDepthExceededBehavior="continue"/>
http://terracotta.org/documentation/4.0/bigmemorygo/configuration/data-life