The =?<charset>?<encoding>?<encodedtext>?= is a means of UTF encoding
the content (i.e., to add Unicode support for e-mail which is
transmitted using ASCII characters). In this case, it was stupid to
"encode" the string using ASCII since ASCII is the base character set.
UTF is often used to encode a string using different characters sets
often but not always to support other languages.
It appears that whomever is sending an encoded string in the Subject
doesn't understand the correct syntax as noted in RFC 2047. Since it is
screwed up, the e-mail client can't handle it. Of course, it's possible
you copied it incorrectly.