I don't think the issue is direct or indirect. DC would have no problem with LHR-FRA-BKK-SIN as an alternative to LHR-SIN, say. In fact, I don't think there is an issue with LHR-DXB-HKG on BA, provided there is availability on CX (tough!)

This is the problem:
(a) PRG-ICN = 5131 miles = base mileage
(b) PRG-MUC-ICN = 5459 miles = base + 6%
(c) PRG-FRA-BKK-ICN = 8131 miles = base + 58%!!!

Whichever way you look at it, routing (c) is taking the p1ss, and highly likely to be above the IATA published maximum permitted mileage for PRG-ICN, particularly as several more direct routes are perfectly feasible

Alternatively, you could break your journey PRG-FRA-BKK Zone 2-Zone 7, and then BKK-ICN Zone 7-Zone 8 - that's only 200k miles in F. That would be perfectly feasible

But I suppose the OP wants to spend Zone-2-Zone 8 miles, with a stopover in Zone 7, and go via the FRA FCT for a bargain 100k return instead?

That's called Trying Your Luck, and I jolly well hope the call centre does not allow the routing