Hi mtwieg,
The Adim is being used. I have putted a 100nf in that pin and I put a constant 4V with a zener circuit to keep the driver working at full power.
There is an application note called AN-9744 which can be found in the onsemi website. It basically explains that when dimming as there is less demand of power the EMI capacitor may not reach 0V ( for ZCD pin) and it can start working on DC mode. It can lead to flickering ( possible noise as well I suppose), to avoid that the AN-9744 recommend to put the HV pin before the diode bridge with two diodes ,then the IC can register every half cycle and it wont get in DC mode anymore. I have tried that and unfortunately it didn't fix the issue.
The Y capacitors are connected between L and earth and N and earth. The neutral and earth connections are shared by both drivers, so there are 4 Y capacitors in total, 2 for each driver. Both drivers never work at the same time ( they are set for different power levels) , but when connecting just one driver ( because the earth and neutral are shared) one Y capacitor of the second driver is also connected because the earth and the neutral are shared by both drivers.
I hope tis make sense. I am planning to remove Y capacitors and separate neutral and earth from both drivers, but after that no idea what else I could think to fix the issue.