After enabling the new 'Sketch_NewSolverAndUI' feature toggle available under File-> Utilities-> Feature Toggle, the new functionality cannot be disabled even if the feature toggle is set to Off.
Solution
This is working As Designed to assist in migrating forward with the new sketch functionality.
Any new sketches in the file will use the new interface and any existing sketches will be given the option to be converted to the new sketch interface even if the feature toggle is set to Off.
Notes and References
Hardware/Software Configuration
Platform: all
OS: n/a
OS Version: n/a
Product: NX
Application: ARCHITECTURE
Version: V1926
Function: FEATURE_TOGGLE
Ref: 001-9948737