DISTINCT ON (field) behaves different inside view definition, appears

Filtering duplicates in this way here works as expected:

woome_app=# select distinct on (ws.id) ws.id, sm.person_id from 
webapp_session ws join sessionmeeting_meeting sm on ws.id = 
sm.session_id where sm.person_id = 45;
   id   | person_id
-------+-----------
  62577 |        45
  62588 |        45
  66598 |        45
  74521 |        45
(4 rows)

Wrapping this query into a view like so:

create view fubar as select distinct on (ws.id) ws.id, sm.person_id from 
webapp_session ws join sessionmeeting_meeting sm on ws.id = sm.session_id;

And applying the same filter to it as above, I get:

woome_app=# select id, person_id from fubar where person_id = 45;
   id   | person_id
-------+-----------
  62577 |        45
  62588 |        45
(2 rows)

i.e. it appears to drop 2 rows it shouldn't. Did I find a bug or can 
anyone explain why this behaviour might be expected?

Frank