You are right. In this case, an IP address shouldn't be changed. SOA record doesn't contain any information about IP.
If you use Plesk Onyx 17.5.3 MU#58/above or Plesk Onyx 17.8.11 MU#22/above, you can find a setting that controls this behavior in “Tools & Settings / DNS Template / SOA Records Template /”. Anyway, I have changed contact information on my Plesk server and check DNS zone settings; only an admin's email was changed.
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I think some other changes were made on your server before. As an example, someone changed DNS zone file by manual through ssh. When you update admin's email, the zone file is recreated by Plesk. In this case, Plesk uses configured DNS records from "Domains / Your Domain / DNS Settings".