It seems elastic has allocated a heap of 32GiB, but uses only 4GiB. Where/why have 32GiB been allocated?

Can you share the elastic part of your docker container? Are you using any overrides?

Last time I tried docker compose, elastic take a lot of RAM !!
You need to limit its RAM usage with mem_limit :

[...]
  elasticsearch:
    networks:
      - backend
    container_name: clearml-elastic
    mem_limit: 2g
    environment:
      bootstrap.memory_lock: "true"
      cluster.name: clearml
[...]

ManiacalLizard2 what happens when ES hits the limit? Does it go OOM, or does the scalars loading just take a long time in the web-ui? And what about tasks putting scalars in the index?

I think ES use a greedy strategy where it allocate first then use it from there ...