Hi,
I'm Catalan, both ways are correct, I looked on the red and I didn't find enough information, I'm pretty sure it is a dialectal difference. In the Eastern Catalan (Girona, Barcelona, French Catalonia and Baleric Islands...) which I speak, the way is "néixer", I have never heard "naixer" near where I live, but I have heard in Valencia (western catalan) "nàixer", in grammar catalan books appears the "naixer" way, because both ways are correct. You can look for a song on youtube "quan jo vaig nàixer" by Raimon, who was an south valencian singer (Xàtiva) and you can hear the valencian accent.