Template is not being used, mapping not working

Elastic Stack Elasticsearch
13

I ended up removing the datetimefield like it was suggested here: https://discuss.elastic.co/t/invalid-format-for-timestamp-field/34793/5

After that, indexing worked fine. Yet the object.tree field is still not being created. If I search

GET beam-*/_search
{
"query": {
"term": {
"object.tree": "/belletristik/"
}
}
}
I get nothing, though I should have a few hundred hits.