Let’s take a look at the mechanics behind Quick Inspiration 2.
This shows the correct solution. But this is not our object of interest here. Why does this work?
First, the effect of the C needs to be explained.
When the C gets put in, the energy of the unwanted configuration increases by 0.6 and the energy of the required configuration by 0.4. This is a difference of 0.2, and since the required configuration is initially more energetic by 0.1, the structure holds.
There is another factor however, and that is the U at position 52. This is crucial. Concentrating on the required multiloop, and varying 37-52 and 53, the following energies are observed:
AU: 2.2 , 2.8, 2.2, 2.6.
UA: 2.1 , 2.3, 2.1, 2.4.
UG: 2.1 , 2.3, 2.1, 2.4.
GU: 2.2 , 2.8, 2.2, 2.6.
(first letter is 37, second is 52, energies are in the order 53 = A, U, G, C)
The key here is in the bold energies: for AU/GU, the energies with C at 53 are 0.4 higher than the energies with A at 53. For UA/UG, they are instead 0.3 higher. Moreover, UA/UG has the unwanted configuration’s energy increasing by 0.3 rather than 0.6. Hence, the latter pairs fail.
Basically, it is a quirk in the Vienna engine that allows this to happen. This has been a bit lengthy, but I hope it helps.