Symmetrical components
In the meantime, the solution was found: the culprits are the delay lines.

The approach I tried was to have instantaneous values rather than magnitude/angle. For this, I tried to mathematically derive the circuit in order to use only a \$-90^\circ \$ delay instead of the \$\pm 120^\circ \$ ("a"), but here is the catch: symmetrical components are meant to be a phasor analysis, and using the delay line as \$-j\$ , for a \$0.1_{DC}+1\angle 0 \$ input (DC separate for better view) I would get \$-j0.9\$ , which explains the erratic behaviour for different DC unbalance levels.

Lesson learned, hopefully remembered when needed. :)