I see the values inside the index as in the posted image.
The string http://127.0.0.1:27336/notify (which I expect to see as a value in the transaction.name field) is split into many individual strings, i.e. http 127.0.0.1 27336 notify.
I think it's a logstash problem, when in a string it encounters special characters, in this case the two symbols : and /, the error occurs.
Using the transaction.name.keyword field for aggregation inside APM, I solved the problem. But I just don't know how to set them.