Why does Sieve behave strangely on a base64 subject line?

Question

I\'m trying to get the message filtered by Sieve by the subject line:

Subject: =?utf-8?B?0K3Qu9C10LrRgtGA0L7QvdC90YvQuSDRh9C10Log0L/QviDQ?=
 =?utf-8?B?t9Cw0LrQsNC