As far as I can tell, from reading the tutorial and different forum entries, the naming of the anchors should be correct.
In a previously exported OTF (exported from Glyphs 2) the marks are not overlapping. I’m working with the latest stable version 3.1.2 (3151).
I found the issue. The dotbelow component has a manually set anchorname (they are set with the anchor button in the info box). Reset it by doing “Glyph > Create Composite”.