-nwa is said to be "a passive suffix" irregularly occurring in the word vanwa "lost" (PE17:63), the word seems to be irregular since the underlying root means "go away" and so vanwa is in a sense a past active participle, *"having gone". Compare PE17:68.
Quenya
-(i)la
suffix. active participle
-la
suffix. active participle
-nwa
suffix. passive suffix; active participle (intransitive verbs), passive suffix; active participle (intransitive verbs); [ᴹQ.] perfective participle
-a
suffix. adjectival suffix
-nwa
passive suffix
This suffix is frequently used to create the adjective form of a noun, especially in the form -ëa for nouns ending in -ë. This function dates back to CE. ✶-ā.