I don't know how many messages have done that today in the past ~7 hours. ~5-ish I guess.
I've been checking "all" once in a while to not miss any.
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Edit
Also - I just noticed that a significant number of messages are entirely missing from "all". I have messages to "5 hours ago" and then there's a huge lacuna to "2 days". I've been posting quite a lot so ~40-ish messages are missing. If not a lot more.
Off topic: Who doesn't like the word "lacuna"?
This seems to happen when someone replies to me and then someone else replies to them. I haven't tested this, but it always seems to be replies with other replies under it that skip my inbox. Idk, happens a bunch
I'm pretty sure it's just once anyone replies to the comment that it will take it off your notifications.