I don’t understand why Apple doesn’t support daisy chaining multiple monitors using DisplayPort Multi-Stream Transport (MST). That would be an elegant solution that also would fit Apple’s style.
They noted that their monitor came with cables that didn't support daisy chaining, when they upgraded their cables to ones that supported 40 Gbit/s then everything was fine.
Probably because their benchmark for displays (>220dpi, plus 10bit HDR in some cases) chews through so much bandwidth that it's not really feasible to squeeze two of them onto a single DP1.4 connection. DP2.1 could do it but Apple doesn't support that yet, and even if they did the monitors would also need DP2.1 passthrough, which also none of them do yet.
MST over DP1.4 only really makes sense for traditional low DPI displays.
They noted that their monitor came with cables that didn't support daisy chaining, when they upgraded their cables to ones that supported 40 Gbit/s then everything was fine.