I think there is something broken with multistops.

Last week I was trying to book an itinerary: BEG-VIE-BKK, SIN-ZRH-BEG. If I input on Google Flight / Matrix / LHG Website all single segments (i.e. BEG-VIE, VIE-BKK etc.) it shows a price in a relatively low booking class (e.g. Z) but it doesn't allow to book my the itinerary. If I search for BEG-BKK, SIN-BEG and choose the exact same segments, I only get the option of much higher booking classes (e.g. C) and the trip actually seems bookable.

The problem of "phantom availability" doesn't seem to be just caused by the LHG website (as it is similar on Google Flight or Matrix), therefore I guess it has something to do with married segments and some sort tariff error? I guess in Multistop booking it uses the avalability of the single segments, while the availability of the whole itinerary is restricted by married segments but this isn't picked up by LHG website / Google Flight / Matrix?