I don’t think this is related to the recipient’s notification preferences. The user whose post I reacted to has the default setting (Notify when liked: “First time and daily”), not “Every time”. But even with that, the issue isn’t about whether they get notified — it’s that in /notifications.json only the very first reaction shows up, and subsequent reactions from other accounts don’t update the merged notification until one of them is removed.
So it looks more like a notification merging/refresh bug rather than a preference setting.