After un-suppressing a Chamfer, the previous Flat Pattern flips from the already selected orientation.
Solution
The selected X-axis has been removed from the Flat pattern. The Chamfer was placed on the selected edge. This is also reported by the grey arrow in the PathFinder, with the informational message "An edge/vertex has been removed and the flat pattern orientation may have changed. The flat pattern should be redefined".
This causes Solid Edge to return to a default position.
Simply re-create the Flat Pattern and it will work again.
Notes and References
Hardware/Software Configuration
Platform: INTL64
OS: window
OS Version: 10_1803
Product: SOLID_EDGE
Application: PART/SHEETMETAL
Version: V219.0
Function: FLAT_PATTERN
Ref: 001-9499942