ACES issue - Posterization in the blacks

Hi, I've been trying to use ACES as a standard for projects in the Post Production company I work for. So far, I've managed to work around the different controls without problems.

But everytime I grade a darker image (or reach close to 0 on the Waveform), some of the information gets weirdly pushed and stretched. The result is either posterization or the impossibility to have softer shadows.

I've tried talking about this with a number of people in the Blackmagic Booth in NAB, and also people in the Post Production World conferences. Some didn't knew what I was talking about, others knew, but had no idea on how to solve this.

Can anyone help me understand what I'm doing wrong? I really want to make it work.

Thanks!

Is this ACEScct?

Is this ACEScct?

happens both with ACEScc and ACEScct

I'm using ODT set to REC709 (as my monitoring) and IDT accordingly to camera (Alexa, Sony in Slog 2 and 3... happens regardless of the source). If I set the IDT to something like REC2020 or P3, the issue diminishes greatly, although I can see it happening a little bit still. But I thought I was supposed to set the IDT in accordance to the source, right?