This is because of the behaviour of that warning:
Flaggers with a recent (past 7 days) flagging history consisting of at least 10 flags where >= 10% of flags were declined will see [that warning] when they flag
You've had 10 flags resolved, 9 as helpful and 1 declined. Since you've been here less than a week, all of those flags are within the past 7 days. So that makes 10% of flags declined in the past 7 days - you've just barely tripped that warning.
You seem to be fine though - you're being helpful. You're just tripping a mechanism not intended for you. That warning exists for other kinds of users who aren't being helpful, to encourage them to pause and think before they flag.
The way out is to wait a few more days, or wait for one or two more flags to get resolved as helpful. The warning is just a warning, and whilst it may be annoying, it's nothing to worry about.
So, carry on! And welcome to Stack Exchange.
Also, regarding the fact was incorrectly declined: even so, the system can't tell the difference, and flag resolutions don't get modified. But it's not really something you have to worry about. As long as you're generally being helpful, the fact you have a declined flag isn't going to mean a thing. (I have several hundred flags on a site I've been active on for ages, and a dozen declined or disputed. Doesn't matter.)