Do you get the same behaviour on a gist or in the live editor? Here’s the source for easy cutting and pasting:
graph BT
t5["t<sub>5</sub>"] --> t2["t<sub>2</sub>"];
t6["t<sub>6</sub>"] --> t2;
t6 --> t3["t<sub>3</sub>"] --> t1["t<sub>1</sub>"];
t7["t<sub>7</sub>"] --> t4["t<sub>4</sub>"];
If it’s buggy there, probably file an issue upstream? If it’s fixed upstream, could probably manually update the component until discourse gets around to an official update.