Outlines getting mangled between versions

A font is built in Glyphs 2, and everything seems fine there. When I open it in Glyphs 3, the outlines of many glyphs get mangled. Example:

You seem to have a cap component that is not working well. Can you send me that file that I can check this?

Sure. I’ll email it.

Please move the “left” anchor in the “_cap.connection” glyph up by one unit.

Well, that solved it — though I don’t understand why. Interesting.

It needs to know what direction to apply the cap. Glyphs 2 was a more limited in how you needed to place the outline. In Glyphs 3, can orient the path as you like. That makes it easier to draw top (or side) caps.