The Snapping functionality does not work while connecting an existing feature to another.
Bug ID Number
October 14, 2015
December 1, 2020
Will Not Be Addressed
The development team has considered the issue or request and concluded it will not be addressed. The issue's Additional Information section may contain further explanation.
This is by design, click the line or polygon feature and move the vertex point to either, vertex (using snapToVertex), edge (snapToEdge), or Point (snapToPoint). Each time this snaps as expected. Entire feature does not snap. Also, snapping in mobile devices is not implemented.