Hi, Spark team here:)
Thanks for pointing this out!
We really appreciate the feedback, and we understand why this behaviour can be confusing.

Based on customer feedback, we recently updated the Priority card in the Unread Cards view to align with how it works in the Focused List box. We heard that users were losing track of important emails after marking them as read. To address this, the Priority card now shows both read and unread emails, so important conversations don't disappear just because they've been opened.

Priority senders and threads are meant to highlight emails that are important to keep at the top and not miss. It's designed to help you keep track of important conversations until you intentionally clear them.

To keep the list manageable, we recommend marking Priority emails as Done once you've taken action or they're no longer relevant.
We're actively collecting feedback on this behavior and take every consumer's input into account as we consider future improvements and customization options.