Resolution
This is intended behavior. Indexing for files is controlled by the portal-ext.property:
dl.file.indexing.max.size
Any file that is bigger than this defined file size will not be fully indexed. Therefore, only a small portion—for example, the first 330 lines in the case of a text file—will be searchable.
This behavior is a function of Lucene/Solr and exists for scalability.