Hello Manuel,
I think that the pool trace information consists of pool allocation and free events (with stack trace information) for events which occur during the trace and a snapshot (state/summary) of pool allocations at the beginning and end of the trace. The snapshots just record statistics for the tag (no stack traces for past events) and that is probably the source of the n/a (not available) entries in the Stack column.
Just investigate the stacks that you do have for that tag and that should lead you to the driver that is using them.
Gary