Thanks Sovichet! I was Zooming with Georg about this in October. The problem is the lookupflag. It needs to restrict the context using UseMarkFilteringSet @belowmarks. Without the lookupflag, as in the first example, any abovemark in between the two belowmarks will break the mkmk attachment. The second example looks like the lookup is restricted to just the dotbelow glyph, so the rules trying to attach it to other belowmarks won’t have any effect.
Related: Skip unhandled marks in ‘mkmk’