Nate's absolutely correct. Notice the endings that are added.
ποιε + ει - Here the long combination ει causes the accent to pull onto the ε.
ποιέ + ει > ποιεῖ - The accent remains on the syllable that was contracted.
ποιε + ε - Here the accent is not pulled toward the end of the word, since the vowel is short.
ποίε + ε > ποίει - The accent remains on the syllable before the contraction.
I think you'll find that it's consistent. The accent is placed on the syllable as if there were no contraction.
Uncontracted: ποιέω, ποιέει, ποιέεις, ποιέομεν, ποιέετε, ποιέουσι(ν)
Contracted: ποιῶ, ποιεῖ, ποιεῖ, ποιοῦμεν, ποιεῖτε, ποιοῦσι(ν)
In each case, the long vowel/diphthong pulls the accent back. This doesn't happen with the short vowel in the imperative singular.