Welcome to forum,
that is normal for the nsg370, well let me clarify, not the popping out, but not going all the way into 1st gear, if you pay attention sometimes going into 1st you can feel a doubke click, when it pops out it just doesn't go all the way into gear, with this transmission and the short shifter might seem like you need to be alittle agressive.
when I got mine with 50k miles I had it pop out acouple times and popes the shifter right out of my hand, but paying attention sometimes you can feel the double click, if it does not go all the way in, or if you wonder, or if it does not go into 1st at all just drop to 2nd then back to 1st, if 2nd a problem, drop to 3rd then 2nd then 1st, the gears might not be synchronization and changing gears helps align.
I changed all fluids when I purchased and trans again at 100k and again at 150k with royal purple.
Every trans shifts different, but tricks help learn best shifting and knowing how to work clutch can make clutch last alot longer.

there are sooo many threads about people scared there trans is out and they just got it, with 1st gear pop out, buy odds are it is just not going all the way in.

any ?'s just ask