Gmail – Why are read emails appearing in is:unread in Gmail

When searching for is:unread in Gmail, there are about 100 email threads in my account that are not bold. (Others are bold, as you would expect.) These emails date back only as far as October 2017, not longer. Here's one example. I am sure I have read it before, and unlike other emails in that search result, it is not bold:

Same thing in Inbox by Gmail:

I can also find these emails using the subject line:

Same thing in Inbox by Gmail:

Interestingly, the email does not show up in a combined search:

Same thing in Inbox by Gmail:

When I click one of these emails, they will no longer appear in is:unread. So there is a simple fix, I am interesting in the reasons.

First question: Why does that read email appear in is:unread? (Could this have something to do with having snoozed the emails in Inbox in the past?)

• Is is technically considered unread because of that snoozing? Then why is it not bold?
• Or is it considered read? Then why does it appear in is:unread? (My highlight of this is when you have only such emails in your search result, you click "More – Mark all read", and Gmail tells you "there are not unread emails"…)

Second question: Regardless of the above: Why does is:unread behave differently when combined with another search phrase? Even without being able to say whether that email is unread or not, is:unread should consistently find or not find it.

My hypothesis for the first question is that Google uses a hacky second unread state that is attached to items that been snoozed and have since reappeared in the Inbox. Technically, each email is not unread, but the fact that the email thread has reappeared may be new. For the user, I believe this snooze-related unread state is visible by a "Snoozed 1 day ago" orange text. However, marking Done/Archiving such an email thread removes that text, while the invisible unread state remains. This may be a bug.

As to the second question, I have no idea why is:unread behaves so inconsistently. is:unread seems to include the second, hacky unread state only when used on it own.