@T3P3Tony thanks for your answer.
Based on your suggestion I managed to have the intended behaviour with the A1 parameter by running on the same line:
M956 P20.0 S1200 A1 F"move.csv" M400 G1 X300 F18000
If I run the same command but with A2 instead, the logged data are pretty much the same, with acceleration and deceleration segments. With A2 I would expect to see only the deceleration phase, am I right?
Btw, I am using 3.5.1.
Thanks for the support!