When I make changes to FAT, then instances between FAT and Regular change, but Regular is unaffected.
When I make changes to SKINNY, then instances between SKINNY and Regular change, but Regular is unaffected.
When I make changes to Regular, then all instances between SKINNY and Regular, and Regular and FAT change.
BUT if I hit Reinterpolate, both SKINNY and FAT remain the same, but the Regular master is overwritten as if it were an Instance.
Surely, the Regular, as a master, shouldn’t change.
The instances interpolate automatically. You don’t need to do anything there. The Reinterpolate command is useful if you like to have some of your masters match the interpolation.
If you search for "Re-Interpolate" (note the hyphen, matching the menu name), the proper page should appear. The mention of that menu item is in section 4.4.1 Working with Layers (page 52 of the Glyphs 2.3 Handbook). fyi.