It is weird that your team can't reproduce it. I'll try to explain more. It seems that in exp 1.3 has more agressive controller sensitivity curve than in stable. It might be happening because added inertia or some other setting interfering cyclic controls. I added picture to show what I mean.

Because of that it makes almost impossible to keep stable movement while turning because there is no middle ground anymore - you either have too little or too much input thus effectively making it more like keyboard input not analog as it supposed to be. This new flight model is realistic only to the fixed wing aircraft, rotary wing should have more sensitivity for micro adjustments as it is in stable 1.2.1 (it isn't ideal but much better than in 1.3 exp). I've added youtube link to comparison video that I made. It should explain more what I mean. You can clearly see that in exp 1.3 flight model makes me jank my sticks because it either gives too much or too little analog input while in stable I can fly pretty smoothly and I don't need to push stick to the maximum positon for turning.

https://youtu.be/_V3nIhyjBNU

i think i can collaborate on this...

i reviewed this problem intensively.... so the problem seems to be the xbox controller analog inputs (all axis) have a not linear curve in the 1.3 experimental builds...

you can recreate the problem anytime as follows (no helicopter needed).... spawn a character then use the analog stick of the xbox controller to turn your character 360°... you have to start with VERY gentle analog stick inputs and increment them VERY VERY slowly from center to full right or full left.... pay attention to the turning speed of your charakter.... around of halfway putting the stick in a direction the turning speed of the character suddenly goes SLOWER than with less stick input and then to the end of the stick input ramps up very fast...

you can recreate this in the helicopter too when the engine is shut off and you stand still on the ground... do the same slow inputs on your analog stick (to the left for example) and pay attention to the in game helicopter joystick... you can see it first moves to the left but then when around half of the stick input on your controller the ingame helicopter joystick literally moves back towards the center before then getting fully tilt to the desired direction....

same with rudder axis.... its a general analog input interpretation problem of the 1.3 builds...

i hope i was able to explain the problem good enough with my words.... just let me know if you have been able to recreate it...