Last year FRA-DOH-FRA in C was as twice as much as FRA-DOH-BKK-DOH-FRA in C.

It's a yield managment thing they call O&D. (origin and destination). They look at the "city pair", in this case FRA-DOH or FRA-BKK and price according to market, demand and competion. DOH is obviously a high yield origin resp. destination.

Same applies to fares ex UK and CH, especially TATL and/or premium classes. An BA tkt FRA-LHR-JFK is generally cheaper than just LHR-JFK.

I remember very well when SR(!) introduced their service to MIA. Inauguration fare ex all Swiss airports 999 SFR, ex all German airports 777 DEM.

Pricing air fares has nothing to do with the average logic we know.

e.g.
Long time ago looking on the city pair BRU-SVO LH had the best connetions by time and duration. There was no direct service those days. So a tkt BRU-SVO on LH was everything but a bargain.

So this happens for long time, even before QR hit the day light.