I’ve done some testing to reproduce this this, and I can – I think it is an issue with Custom Line Chart Widgets on MQTT devices on our end, nothing wrong in your code.
In my test, it looks like Line Chart widgets created via Add New > Device / Widget > Custom Widgets > Line Chart have this issue, while the widgets auto-created via an MQTT data publish and then accepted on the dashboard do not. This should allow a workaround, at least.
Try changing the ‘Channel 3’ widget via its settings dialog to Line Chart type. Then change the ‘ch3’ widget (the one with no data in the graph) to a Value or Gauge type. That should allow you to see the same data twice on your dashboard, both as value and chart, without any broken chart.
I’ll get the bug logged with our dev team, thank you for the detailed screenshots and information!