Hello again,
The 3rd output is never using our technology so whether it works and fixing it, is not under my control, unfortunately. The chip we make that Dell uses in the box only has 2 simultaneous outputs possible.
When using 2 displays (DPx2, DP+HDMI) it all goes through us because of internal switches, when you add the 3rd display, both DP are connected to our chip, and the HDMI is driven by the laptop through a converter (like your USB-C to HDMI standalone converter you tried, because the laptop outputs DisplayPort Alt-Mode and standard USB, not HDMI Alt-Mode) and that's when it starts the flickering.
I don't understand your case of DP->HDMI active adapter.
A passive adapter means it's not really DisplayPort, it's HDMI signals disguised in DisplayPort connector (called
DisplayPort++ or Dual Mode).
An active adapter means the adapter is generating an HDMI signal itself, so it's not driven from the dock or the computer.
Kind regards,
Alban