Hi there,
As far as I understand, event.module is not being added to each document originating from Winlogbeat (starting from v8.0.0) that uses Winlogbeat modules.
This field was previously seen from v7.4.0 onwards, where the field was added in the Winlogbeat module's JavaScript file. Winlogbeat modules changed from 8.0.0 onwards to use Elasticsearch Ingest Node for processing.
In the winlogbeat-[beats_version]-[module_name] ingest pipeline, event.module is not seen to be added. Here is a snippet of the ingest pipeline, where all parts of the pipeline that handles the event.X field is seen (we do not see event.module):
I do not see it being added in the winlogbeat-[beats_version]-routing ingest pipeline too.
Here is a snippet of a Winlogbeat document (after it had been parsed by the Sysmon ingest pipeline):
References:

