Hi Georg, this isn’t causing me any issues because the exported fonts are good, but this is strange. I’ve used the macron as the bar and Auto Alignment is turned on by default, Just surprised that the preview is different from the glyph and exported fonts.
Which app version is this? Can you see if this is fixed in the latest cutting edge version?
I would not use the macroncomb here, rather a _part glyph that you could also reuse inside macroncomb.
Version 2.4.1 (983), I dare not use the cutting edge version - too many problems with those in the past.
I’ve decomposed the Glyph now. cheers Erich 
Two things: Could you still try (a copy oft) the font in the cutting edge version? And if that gives the same result, send me the .glyphs file?