These are longitude, latitude, height of your points.
-83.60630603436458, 49.77089753350893, 549155.5299375599
-83.60638176005732, 49.7709088949907, 549163.2629633523
Reason is that height of your points is too high or lat/lon of your point is almost same.
To elaborate that: You’re looking at a line that is ~10 meters long, and at a height of ~550 kilometers. The camera interaction speed in CesiumJS depends on the height of the camera over the ground. (You can quickly navigate over the globe, but close to the ground, the navigation should be slower). So even the smaaaalest movement will move that 10 meter line waaay out of sight.
You can center the camera at one end of the line, and/or make the zooming speed slower:
(I’m not sure what exactly should happen when you navigate in the original sandcastle…)