Philomen Probert, A New Short Guide to the Accentuation of Ancient Greek
(2003) sec. 286, p. 148 writes: "A perispomenon full word retains the circumflex on its final syllable before any enclitic, and the enclitic again has no accent . . . " She gives examples of an unaccented disyllabic enclitic following a perispomenon. She notes that most of our evidence supports this, and standard editorial practice follows this rule. But she qualifies this by citing, without explaining, two secondary sources.
This is the standard rule. There may be grounds for questioning it, but this is what you'll find in modern editions of Attic authors.
Chandler, A Practical Introduction to Greek Accentuation
, 1866, sec. 972, p. 280 cites Charax (an ancient grammarian) in support of this rule, noting that the 19th century scholar J. G. J. Herrmann argued against the rule.http://en.wikipedia.org/wiki/Johann_Gottfried_Jakob_Hermannhttps://archive.org/details/cu31924021602218
In short, the most widely accepted rule is that a disyllabic enclitic following a perispomenon word reamins unaccented, but there may be grounds for questioning this rule. If you're attempting to master Greek accentuation, I would suggest following this rule, which is what you'll find in modern texts, but you might explore this further if you have access to a reference library. I would recommend Probert's book, too.