Thread pitch error

Hi all,
It's my first post here, hope in the right section.

I have a lathe, Padovani labor 165 with Fanuc O-TC, located in Italy (50 Hz and three phases). I bought it last year and now I'm occur a big error.

I need to make a trapezoidal thread with 12mm of pitch and when I use a G76 comand with F12 it make a pitch of only 6mm
With this program:

(O1001
N10 G98 G18
N11 G21
N12 G92 S2000
N13 G28 U0.

(THREAD5)
N14 T0202
N15 G54
N16 M8
N17 G99
N18 G97 S558 M3
N19 G0 X57. Z5.
N20 G92 S2000
N21 G96 S100 M3
N22 G0 Z-0.68
N23 G76 P020015 Q50 R0
N24 G76 X40 Z-44.5 P4000 Q50 F12.
N25 G0 X57. Z5.
N26 G97 S558 M3

N27 M9
N28 G28 U0. W0.
N29 M30
)Generate with Fusion360

so I don't know if my machine is allowed to use G98/G99 or G94/G95, until today I haven't got any problem like this... and I always used G99/G98

I don't know how to solve this problem, any tips is accepted!
Thank you
Emanuele

Do NOT use CSS (G96) when threading.
do it all using a fixed speed (G97) as a changing speed alters the start of thread timing... so remove G96 S100

Check manual if approach angle of 15° is allowed. so controls may use only certain angles ( P020015 try P020000 to see if that alters the result )

A long with superman's sugestion
slow the spindle speed down my sugestion would be about 100-150 rpm to start
Your g96 is causing the rpm to be about 725 rpm
how good can your machine handle high feed rates you are using ?

725 rpm x 12mmpr=8700 mmpm which is pretty quick for threading