1st hear pop out is a common occurence on the nsg370, mine scared the crap out of me when it 1st did it, 120k miles ago.

It is learning how to pay attention and driving this trans.
If you pay attention closely you can feel a double click going into 1st gear, if it doesn't, it is not all the way in.

If in question if it is all the way in, while stopped, shift to 2nd then back to 1st or 3rd, 2nd then 1st.

When coming to a stop, before car is all the way stopped slide into 1st.

Several other things can do or watch out for.

I have 178k miles on mine and original clutch, and mine 1st happened to me around 60k mikes.

I change the trans oil with royal purple every 50k miles