Segments_per_tier policy not enforced?


I am struggling to keep my number of segments per shard low without having
to perform some forced _optimize. I thought setting "segments_per_tier" and
"max_merge_at_once" with a low value would limit the number of segments,
but no matter what I set my shards always end up with about 15/20 segments.

I indexed a few millions of documents on a single node, merges happened,
but in the end there are still about 18 segments per shard, even with
segments_per_tier set at 2. And the log shows that no merges have been
performed since yesterday.

Am I missing something ? There are my settings, my segments layout and a
piece of the merge log: