It seems there are several disjunctions between wgContentNamespaces and wgNamespacesToBeSearchedDefault. The latter is what is reflected as "Content" in the Special:Search page (see T170473 for more info).
After consulting with the community, it's been decided to add the following namespaces to $wgNamespacesToBeSearchedDefault for Bengali Wikisource:
- Namespace 100 (Author)
- Namespace 106 (Portal)
- Namespace 114 (Translation, added in T114623)