Relations (constraints) that are manually added in the new Sketcher do not stay, and can disappear when adding more dimensions.
Solution
In the new Sketch, relations (formally known as constraints) are not meant to be permanent. They simply move the curves into place. The manually added relation is not enforced. As a workaround, try turning on 'Create Persistent Relations', then manually create the relation/constraint. This will help the new relation "stick".
Please note that 'Create Persistent Relations' is not on by default and may be hidden.
Notes and References
Hardware/Software Configuration
Platform: all
OS: n/a
OS Version: n/a
Product: NX
Application: DESIGN
Version: V1953
Function: SKETCHER
Ref: 001-10038922