Top anchor added to bottom mark

When exporting instances for a font I’m working on, Glyphs 2.5.2 (1158) adds a top anchor to a glyph that’s really a bottom mark.

The glyph is nyo-khmer.below.2, a subjoined form for a Khmer consonant. It has no outline of its own, but uses the glyph tha-khmer.below.2 as a component, rotated by 180° and offset. It also has anchor points _bottom, bottom, and bottomright. tha-khmer.below.2, in turn, has an outline and the same anchor points.

The generated features.fea file has the following expected lines relating to this glyph:
markClass [nyo-khmer.below.2] <anchor -188 0> @mark_bottom; # top level
pos mark [nyo-khmer.below.2] <anchor -188 -195> mark @mark_bottom; # lookup blwm_khmer_bottom
pos mark [nyo-khmer.below.2] <anchor -37 -198> mark @mark_bottomright; # lookup blwm_khmer_bottomright

But then it also has this unexpected line in lookup abvm_khmer_top:
pos mark [nyo-khmer.below.2] <anchor -182 -72> mark @mark_top; # lookup abvm_khmer_top

As a workaround, I found I can make the unexpected line go away by requesting to decompose the glyph during export.

When you rotate a mark, the anchors are switched. If you have a component of commabelowcomb and rotate it (to put it on top of the lowercase g), the _bottom anchor is switched to _top.

This is what’s happening here. I see if I can fix this.
To be sure, you could make a third glyph that only has the outlines and the use that as a component in glyphs.