Searches within dashboards are run in "fast mode", which means that only those fields that are explicitly referenced within the search are available as selected fields in the result set.
To ensure that you pass the necessary fields from a global search down to the post process searches, you will need to use the "| fields" command with the list of fields needed. As a shortcut, you can also use "| fields *" to pass all fields as selected fields.
So your search could look something like this:
<searchTemplate>index=_internal log_level="ERROR" | fields *</searchTemplate>
... View more