In the discussion Custom LSB & RSB for Zero Width Mark Attachment I learned that marks are automatically reduced to a zero-width where the right edge becomes the center of the zero-width glyph.
This behavior has changed in Glyphs 3; now the glyph is collapsed onto the left edge.
The new behavior:
Here is the test-font I used for the screenshots: Test.glyphs (6.8 KB)
If this new behavior is intentional I can change my glyphs.