If you're at a stop, clutch in, engage first gear, clutch out, and it pops out of first gear, it means that the gear engagement dogs aren't fully seated. This could be due to not using enough force to engage the gear, or it could be a synchro issue, or a trans fluid issue.
My 2017 Corvette Z06 7MT did the first gear popout a couple times, only when cold, and only if I didn't engage first gear with some serious authority. (I bought it in February with 17K miles.) Per the Corvette world's brain trust, I installed Redline D4 trans fluid and that seems to have resolved the issue.
My '23 5BW, bought new, has never popped out of any gear, and if it did, I would be very worried. At the very least, I would notify the dealer and ask them to document the issue while the car is still under warranty, so that if it gets bad enough to get GM to take action, it would hopefully be covered as an existing issue even if the car was out of warranty.