This tooltip was recently changed back on April 17th. It was felt that the "noise" descriptor was somewhat vague. Moreover, "unconstructive" was always a valid flag reason, but it was merely grouped somewhere between "noise" and "offensive" in the flag reasons. As such, in the interest of being more explicit about it, "noise" was replaced with "unconstructive".
The utility of flagging hasn't actually changed, it's just somewhat more explicit about it.
As far as comment flags in general go...
Comment flags are not often handled by moderators except in egregious cases. Rather, it's usually the community accumulation of flags that results in the destruction of such comments. If the required number of comments is not accumulated in 4 days time, the comment flag will age away and you'll take the flag weight drop.
As such, when you lose flag weight for a comment flag, it simply boils down to a case of not enough people agreeing with you on that particular judgment. One errant flag is not a huge dent, and since comment flags are only worth half the flag weight, just keep going forward.
Don't think too hard about every single flag you cast - think more about the long term patterns you develop. If you start to consistently lose flag weight, that's when you should be concerned. But if you only lost a single step, then shrug it off as a one-time mistake.