OverlayNG difference operation moves points in outer edge

Stepping through the code the change in point locations occurs in GeometryOverlay in the JTS code base. isOverLayNG is set to true so OverlayNGRobust.overlay is used. The two input geometries are:

POLYGON ((412023.08 92158.29, 412021.68 92161.6, 412022.58 92161.96, 412021.45 
92164.58, 412022.19 92165.38, 412021.43 92167.13, 412018.32 92165.86, 412016.28 
92165.03, 412019.75 92156.92, 412023.08 92158.29))

and a negative (-0.5m) buffer of it:

POLYGON ((412022.4 92158.6, 412020 92157.6, 412016.9 92164.8, 412021.2 92166.5, 
412021.6 92165.5, 412021.1 92164.9, 412021 92164.8, 412021 92164.7, 412021 
92164.6, 412021 92164.5, 412021 92164.4, 412021.9 92162.2, 412021.5 92162.1, 
412021.4 92162, 412021.3 92162, 412021.3 92161.9, 412021.2 92161.8, 412021.2 
92161.7, 412021.2 92161.6, 412021.2 92161.5, 412021.2 92161.4, 412022.4 
92158.6))

are differenced to give a bordered polygon. which results in:

POLYGON ((412019.8 92156.9, 412016.3 92165, 412018.3 92165.9, 412021.4 92167.1, 
412022.2 92165.4, 412021.5 92164.6, 412022.6 92162, 412021.7 92161.6, 412023.1 
92158.3, 412019.8 92156.9), (412021.2 92161.4, 412021.2 92161.5, 412021.2 
92161.6, 412021.2 92161.7, 412021.2 92161.8, 412021.3 92161.9, 412021.3 92162, 
412021.4 92162, 412021.5 92162.1, 412021.9 92162.2, 412021 92164.4, 412021 
92164.5, 412021 92164.6, 412021 92164.7, 412021 92164.8, 412021.1 92164.9, 
412021.6 92165.5, 412021.2 92166.5, 412016.9 92164.8, 412020 92157.6, 412022.4 
92158.6, 412021.2 92161.4))

Some of the points in the outer polygon are moved for no reason.

Reverting to isOverlayNG=false solves this issue.

Start in GeometryOverlay, where isOverLayNG selects OverlayNGRobust.overlay, and reproduce the difference using the two polygon inputs in the issue. Compare the OverlayNG result with the isOverlayNG=false result, focusing on the changed outer-ring coordinates. The fix is complete when OverlayNG preserves the unaffected outer points while still producing the bordered polygon.