The reason I say 'should' is that papers written in the present tense are also occasionally found.
Literal
'Should' [called-というの][topic-は], present-tense [in-で] written papers [also-も] occasionally-seen [reason-からです].
散見される 'to be occasionally seen/found' is a formal expression — 散 'scattered' + 見 'seen.' 論文 'academic paper.' The sentence explains a metalinguistic choice — why 'should' rather than 'must' is used for a grammar rule that has exceptions.