Approaching NDL alert pop-out at my second dive but NDL shows 24 mins.

Yeah, it is "normal" and makes no sense.  Mine usually shows around 30 minutes but then never again.

I've gone to zero and then into deco (I knew it was happening and was fine with it) but I only ever got the one warning at around 30 minutes.  The warning really is of no value at all.

I've noticed this a bunch also with my NDL alert going off when I still had 30+ minutes left. I might be wrong but after trying to keep track of when it went off on multiple dives it seems like the alert triggers not based on actual NDL time left but based off the N2 graph on the left side of the face. It seems that it triggers as soon as the N2 graph reaches the yellow/orange part of the graph regardless of how much actual NDL time you may have left. If that is the case Garmin should change that to where it triggers off the actual remaining NDL time left.

Ya, I agree so strange really wish the manual have for information. 

Oh, that's an interesting observation I will have to test this out than. Thanks

Disregard the warnings, just watch the time remaining closely.  If you’re getting close to NDL go slightly less deep, even a few meters, which makes a big difference.  That’s assuming you’re diving with an organized group that cares about deco mode.  If not, then you know what to do.