message-id is (or at least was ~5-10 years ago) only required for automated mails, i.e. if you send a mail which looks mostly the same to a lot of users

so if you (ab-)use protocols for mail clients to send automated mails they might very well not add Message-Id. In general many mail providers have both a way to send a mail where "they do some clever parts" (like adding Message-Id) and interfaces where they mostly just send what you give them. It's possible that they migrated from one solution which did automatically add Message-Id to one using interfaces where it doesn't happen without realizing that there is a mismatch in this solutions do implicitly add.