There are many spam emails sent with the same subject but use a different "From" addresses. They may appear the same as earlier blocked emails but the "From" address is different. The "From" address is added to the spam block list when you use the "Spam" swipe action. The spam block list is then used to check the "From" address from any new emails that you receive.

You can check any emails that are not marked as spam but should be. Tap to select the email and tap on the ⓘ button or choose "Details" in the action menu.
Open the 'Spam Suggestions' section to see suggestions on blocking emails like the one you received. More

You can create email rules if you want to block spam email by subject or other content. These rules can contain multiple keywords in the subject, from address, message content or other fields. Rukes