I don’t know whether that’s the case here, but the most common cause for this behavior is not training all the mistakes. A lot of people will train some but skip others, or not see them because they get moved by other Mail rules, or delete them on their phone instead of moving them to the TrainSpam mailbox or training them when they get back to their Mac. Not correcting mistakes is like telling SpamSieve that you do want to see more of those messages.
Since you’re a longtime user, it’s also possible that SpamSieve’s training data has gotten bogged down with stuff that’s no longer relevant, and it’s time to reset the corpus and retrain with a smaller number of recent messages.
But before jumping to any conclusions, I recommend following the instructions on this page to make sure everything is still set up correctly and (if it seems to be) to send us a diagnostic report to investigate further.