Pretty sure this information comes across via Log Exporter, though I haven't checked.
The fact you can search and find that specific result is a good sign the necessary field is indexed, there just may not be a way to refer to that specific field.
I believe the schema files are in $RTDIR/solr/configsets and you can see what fields are indexed by their internal name.
Theoretically, these can be modified as well, but we don't support this and doing so can cause a significant performance degradation.
An RFE through the local office would be required to confirm if this could be done.
Meanwhile, there is an interesting tidbit in the R81.10 EA release notes that is relevant: "The Solr functionality is replaced with a PostgreSQL database to improve the stability and performance of the Security Management Server."
Solr is used not only for logs, but for searching some parts of the Security Management database as well.
Solr will be removed in R81.10 for non-logging functions, but it will still be used for logs.
In addition, Solr was upgraded as part of R81, which led to some performance/stability improvements.
(Edited statement related to Solr removal on 24 June 2021)