"SORT-STRING" or "X-ALT-NOTE;FMTTYPE" appearing randomly in contacts

Several contacts have weird text inserted in their Suffix field (the field where Jr. or Esq. or III normally goes). The junk text always starts “SORT-STRING:” followed by the person’s last name, first name, etc.

Other contacts have “X-ALT-NOTE;FMTTYPE=text/html…” in various other fields.

I sync emClient with two iOS devices, but no other PCs. The text was definitely not there before using emClient. The problem also grows over time: more and more contacts get their field corrupted over time.

Any ideas?